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To help answer this question, I did some research into what how many user accounts share the same username or email. While usernames turned up a bunch of common first names, something thats expected, email accounts turned up many accounts with the same exact email hash. I thought this was important enough to get its own topic.

First, here's a sample of what I pulled

EmailHash - NumOccurrences
5698d4707354e0b9e4ba6f22a0be84d8 - 77
a432ac3e33b49f53223cf795d31577e8 - 69
04b942ca7001ac7179aa14bb3d590878 - 68
415a1da470f0a73906b6833dbf773cad - 61
bede3927cdd54a89c3fddd047af0aef1 - 55
c1fa7618ad66b58dffd2b3bf71d2ae8a - 51
be21766f09e0e5fd3a2f8cc721ceba2e - 46
5d83b94b455417fe0cf6a03226172146 - 42
ce49626bea8d1e4949e87cacce662153 - 41
34640dc7d06dae9841470272b121a452 - 38
06451734c4c13fb69981df88dad76e64 - 37
0ea032f0661d091ab4d53e7d214f4d68 - 37
e91cef26ff569af3fa04027d1b5c8a01 - 37
10f59ffb03e3d661037b1312c0d0fb2c - 36
a6bb875992f2bea471b788611d3afc40 - 35
5810bda19603579e6586799528789d8b - 34
1fb5f7dc722758d9a8e38d36debb7218 - 32
196db543a462291fe5497917ebda5736 - 31
edf0be6f27738ee7344f767f8e066274 - 31
f24da0fb92ce004520e876355912f910 - 31
dc0171026bdae4950c91b2c147cd41ba - 30
297f83e02470bf8b797470b126141087 - 30
980b3fc8b51bfc7e8fc01576ea240481 - 30
0e5be616468b4bd5e535cc75a38da179 - 29
cf2e79eb6971da96eb26b3fc5c9514cd - 29
bdc17f5a771a988a9b454d4cafd3780a - 28
7e31c1742a331606a330ba3ac3c043c9 - 28
1d53be99f50b32d9f28cd586610aac31 - 27
c85afe296810ecd9127a58247e739996 - 27
a69ae3e7d0277ee982fcd653e5e76a3e - 26
1708a313f4edf8349b14964a9259c302 - 26
1c1079a7b3e0185c015f1c7d5e5e6bc4 - 25
7545330f5d2295050d036ed33bd6382f - 25
ec4338bd089a8f6446ef2bfa4a5f9a70 - 25
a04a3471c81481d9f108d66280d7effc - 24
17a11a6d92477ef0e8e9a2e0359bacc6 - 24
710834fe2cdec6362c9636b1b230ceaa - 23
7c97003be330bb45cd015ef2754e9d00 - 23
437e858d9d128ed9e67d50cbb9b0ff24 - 22
c6dacdcf88a313c0f09e3b535d687b04 - 22
fcd5c17e9016adf856259b6273331168 - 21
fdb62ecb445536e1a8e981f4f4332c42 - 21
3f009d72559f51e7e454b16e5d0687a1 - 21
4b57d3cae7e0033819c3ed85d1b1c865 - 21
4eab895c56ce28ced600ee72162ee0ba - 20
2ac71aa212ba7b8be169266cad29ff65 - 20
edc0d7137b55fafed121a336eea571c7 - 20
f650a086561b4ec5539449bfabc386bb - 20
c838724626bd528f2a8df4ff3d7c5a78 - 20
bec51d7b9c255206b62173a0e58cebb8 - 19
b7cf765a1e1158388b6de8cc86cc2075 - 19
f8e6d4455b3057e6bae305534d80f68a - 19
32d18237527880c030f59f79e6b59ee7 - 19
41715cd2391d035e7a6a3fa8fe52e326 - 19
879628a231815611d2275bdc5c982788 - 19
58d76b633d7adabcfb62a702f9623518 - 19
47e915d7110c0a6a9f403350bd024de5 - 18
b7436e88c1eb190f4ce173e544f0982a - 18
a8d4f0e50c32679eb7a9d08196c71917 - 18
df22200e12d539174dcff9334697460b - 18
e048a299c7ddc828e89f1b5fb166a2a6 - 18
ea0cc83b36088dd5b4bdac127b5068f2 - 17
d802d8212ed5139e303a577f04f49ac3 - 17
d7e1cc08494d0d72d5c73f0ecf919f58 - 17
bf3e5d1d64e8efc3eab5bda20fec71b7 - 17
f3ada405ce890b6f8204094deb12d8a8 - 17
0bbd01f43ab82cf4366f23c5445e8c20 - 17
0011078d69f2a472d19077d4324fd0fe - 17
5789d05781cd58a8954ac23941af7a4b - 17
6dffd179717cacdde5bd8c9b78d014f7 - 17
65ab5ce0caf15fb04b6a2cce7a55ce5b - 17
6c0b5bef9ad73ee989791278d595dfa9 - 17
820d0e4ee14e986a44d33782ca852f51 - 16
84f0e32d74c7077c56fae7775e0eb509 - 16
22f3d395338b76850eea9bfd6e7c9be0 - 16
412f7a08e1f7952c2da13f1d862e31c2 - 16
f4b3a202f86bce6b3598f6c9143ac908 - 16
bbba7362d4bd9b00b302b5bc90c4e4d5 - 16
c71bb148768b62da18f287c8d14ff2fe - 16
dfc3b2964efc6f152b4fcef98c94a4a9 - 16

This is either the most shocking example of hash collisions I've ever seen, or we have a bunch of accounts sharing an email account.

Shouldn't typing in an email account that already exist in the db be a red flag that the account is a duplicate? Shouldn't there be some kind of notification to mods that the accounts should be merged?

As an example, lets take the first hash off the list and see some info on who's using it

Id     Reputation EmailHash                        CreationDate        DisplayName LastAccessDate      WebsiteUrl Location Age  AboutMe Views UpVotes DownVotes 
------ ---------- -------------------------------- ------------------- ----------- ------------------- ---------- -------- ---- ------- ----- ------- --------- 
190337 6          415a1da470f0a73906b6833dbf773cad 2009-10-15 4:21:21  jim         2009-10-15 4:21:21  null       null     null null    4     0       0         
190404 3          415a1da470f0a73906b6833dbf773cad 2009-10-15 7:28:14  Jim         2009-10-15 7:59:17  null       null     null null    0     0       0         
192005 1          415a1da470f0a73906b6833dbf773cad 2009-10-18 13:01:10 Jim         2009-10-18 14:05:00 null       null     null null    1     0       0         
192287 1          415a1da470f0a73906b6833dbf773cad 2009-10-19 8:19:08  Jim         2009-10-19 8:51:23  null       null     null null    0     0       0         
193148 11         415a1da470f0a73906b6833dbf773cad 2009-10-20 14:14:35 Jim         2009-10-20 14:49:08 null       null     null null    0     0       0         
194754 6          415a1da470f0a73906b6833dbf773cad 2009-10-22 17:07:41 jim         2009-10-22 17:38:04 null       null     null null    0     0       0         
194989 4          415a1da470f0a73906b6833dbf773cad 2009-10-23 0:12:34  Jim         2009-10-23 0:12:34  null       null     null null    4     0       0         
196222 11         415a1da470f0a73906b6833dbf773cad 2009-10-25 16:04:03 JimS        2009-10-25 17:47:51 null       null     null null    0     0       0         
199839 1          415a1da470f0a73906b6833dbf773cad 2009-10-30 18:10:06 Jim         2009-10-30 19:14:01 null       null     null null    3     0       0         
200386 8          415a1da470f0a73906b6833dbf773cad 2009-11-01 5:31:44  Jim         2009-11-01 6:34:11  null       null     null null    1     0       0         
203119 6          415a1da470f0a73906b6833dbf773cad 2009-11-05 2:48:13  Jim         2009-11-05 4:19:54  null       null     null null    2     0       0         
220291 1          415a1da470f0a73906b6833dbf773cad 2009-11-28 0:08:17  Jim         2009-11-28 0:44:17  null       null     null null    0     0       0         
220322 6          415a1da470f0a73906b6833dbf773cad 2009-11-28 2:45:10  Jim         2009-11-28 2:45:10  null       null     null null    0     0       0         
221709 1          415a1da470f0a73906b6833dbf773cad 2009-11-30 23:10:17 Jim         2009-12-01 0:07:52  null       null     null null    0     0       0         
222393 1          415a1da470f0a73906b6833dbf773cad 2009-12-01 20:08:23 Jim         2009-12-01 20:08:23 null       null     null null    0     0       0         
237999 3          415a1da470f0a73906b6833dbf773cad 2009-12-24 1:58:08  jim         2009-12-24 3:33:56  null       null     null null    0     0       0         
238149 1          415a1da470f0a73906b6833dbf773cad 2009-12-24 9:37:46  jim         2009-12-24 10:08:48 null       null     null null    0     0       0         
238460 3          415a1da470f0a73906b6833dbf773cad 2009-12-25 3:13:48  jim         2009-12-25 3:13:48  null       null     null null    1     0       0         
240601 1          415a1da470f0a73906b6833dbf773cad 2009-12-30 2:46:31  jim         2009-12-30 3:17:26  null       null     null null    0     0       0         
241904 3          415a1da470f0a73906b6833dbf773cad 2010-01-01 12:14:23 Jim         2010-01-01 12:45:28 null       null     null null    0     0       0         
242248 3          415a1da470f0a73906b6833dbf773cad 2010-01-02 10:23:28 Jim         2010-01-02 10:23:28 null       null     null null    1     0       0         
242323 11         415a1da470f0a73906b6833dbf773cad 2010-01-02 15:03:55 jim         2010-01-02 15:39:51 null       null     null null    1     0       0         
244524 3          415a1da470f0a73906b6833dbf773cad 2010-01-06 6:47:32  Jim         2010-01-06 7:18:26  null       null     null null    1     0       0         
245441 3          415a1da470f0a73906b6833dbf773cad 2010-01-07 11:01:46 jim         2010-01-07 11:01:46 null       null     null null    0     0       0         
245829 1          415a1da470f0a73906b6833dbf773cad 2010-01-07 19:07:07 Jim         2010-01-07 19:37:57 null       null     null null    0     0       0         
246489 11         415a1da470f0a73906b6833dbf773cad 2010-01-08 15:10:22 jim         2010-01-08 21:50:25 null       null     null null    9     0       0         
247554 6          415a1da470f0a73906b6833dbf773cad 2010-01-10 16:58:31 Jim         2010-01-10 16:58:31 null       null     null null    2     0       0         
247596 3          415a1da470f0a73906b6833dbf773cad 2010-01-10 19:06:40 jim         2010-01-10 19:06:40 null       null     null null    1     0       0         
250994 1          415a1da470f0a73906b6833dbf773cad 2010-01-14 18:57:31 Jim         2010-01-14 18:57:31 null       null     null null    0     0       0         
251004 8          415a1da470f0a73906b6833dbf773cad 2010-01-14 19:08:05 Jim         2010-01-14 20:13:03 null       null     null null    2     0       0         
252016 20         415a1da470f0a73906b6833dbf773cad 2010-01-16 4:40:38  jim         2010-01-16 4:40:38  null       null     null null    3     0       0         
258329 21         415a1da470f0a73906b6833dbf773cad 2010-01-25 10:37:58 Jim         2010-01-25 10:37:58 null       null     null null    1     0       0         
272167 1          415a1da470f0a73906b6833dbf773cad 2010-02-12 21:55:39 Jim         2010-02-12 21:55:39 null       null     null null    1     0       0         
272224 18         415a1da470f0a73906b6833dbf773cad 2010-02-13 1:12:29  Jim         2010-02-13 1:50:30  null       null     null null    1     0       0         
272263 3          415a1da470f0a73906b6833dbf773cad 2010-02-13 4:12:52  Jim         2010-02-13 4:12:52  null       null     null null    0     0       0         
272668 3          415a1da470f0a73906b6833dbf773cad 2010-02-14 6:20:57  jim         2010-02-14 6:51:06  null       null     null null    1     0       0         
278598 33         415a1da470f0a73906b6833dbf773cad 2010-02-22 10:26:42 Jim         2010-02-22 10:26:42 null       null     null null    0     0       0         
279087 1          415a1da470f0a73906b6833dbf773cad 2010-02-22 23:20:23 Jim         2010-02-22 23:20:23 null       null     null null    0     0       0         
279928 8          415a1da470f0a73906b6833dbf773cad 2010-02-23 23:13:09 jim         2010-02-23 23:13:09 null       null     null null    0     0       0         
280228 13         415a1da470f0a73906b6833dbf773cad 2010-02-24 10:30:40 jim         2010-02-24 10:30:40 null       null     null null    0     0       0         
280946 8          415a1da470f0a73906b6833dbf773cad 2010-02-25 6:00:13  jim         2010-02-25 6:00:13  null       null     null null    0     0       0         
281263 18         415a1da470f0a73906b6833dbf773cad 2010-02-25 14:12:51 jim         2010-02-25 14:12:51 null       null     null null    0     0       0         
281850 5          415a1da470f0a73906b6833dbf773cad 2010-02-26 7:02:24  Jim         2010-02-26 8:27:47  null       null     null null    0     0       0         
282492 16         415a1da470f0a73906b6833dbf773cad 2010-02-27 1:35:45  Jim         2010-02-27 2:07:00  null       null     null null    0     0       0         
283309 1          415a1da470f0a73906b6833dbf773cad 2010-03-01 0:18:19  Jim         2010-03-01 0:18:19  null       null     null null    1     0       0         
284232 13         415a1da470f0a73906b6833dbf773cad 2010-03-02 8:49:32  jim         2010-03-02 9:56:21  null       null     null null    0     0       0         
284330 11         415a1da470f0a73906b6833dbf773cad 2010-03-02 10:55:47 jim         2010-03-02 12:49:14 null       null     null null    11    0       0         
284504 1          415a1da470f0a73906b6833dbf773cad 2010-03-02 15:04:32 jim         2010-03-02 15:04:32 null       null     null null    0     0       0         
286153 8          415a1da470f0a73906b6833dbf773cad 2010-03-04 10:14:29 jim         2010-03-04 11:18:37 null       null     null null    1     0       0         
286224 1          415a1da470f0a73906b6833dbf773cad 2010-03-04 12:03:18 jim         2010-03-04 13:45:21 null       null     null null    4     0       0         
286942 1          415a1da470f0a73906b6833dbf773cad 2010-03-05 8:30:41  jim         2010-03-05 8:30:41  null       null     null null    2     0       0         
288021 13         415a1da470f0a73906b6833dbf773cad 2010-03-07 1:46:27  jim         2010-03-07 2:17:37  null       null     null null    1     0       0         
288081 8          415a1da470f0a73906b6833dbf773cad 2010-03-07 5:48:09  jim         2010-03-07 10:43:54 null       null     null null    1     0       0         
290176 1          415a1da470f0a73906b6833dbf773cad 2010-03-10 3:26:50  Jim         2010-03-10 4:01:27  null       null     null null    0     0       0         
309660 1          415a1da470f0a73906b6833dbf773cad 2010-04-06 2:21:26  jim         2010-04-06 4:35:21  null       null     null null    1     0       0         
406873 3          415a1da470f0a73906b6833dbf773cad 2010-07-30 15:24:26 Jim         2010-07-30 15:59:43 null       null     null null    1     0       0         
456672 3          415a1da470f0a73906b6833dbf773cad 2010-09-23 21:18:29 Jim         2010-09-23 21:33:40 null       null     null null    0     0       0         
456884 1          415a1da470f0a73906b6833dbf773cad 2010-09-24 4:12:47  Jim         2010-09-24 4:12:47  null       null     null null    1     0       0         
457736 3          415a1da470f0a73906b6833dbf773cad 2010-09-24 20:58:12 jim         2010-09-24 21:29:15 null       null     null null    1     0       0         
457890 1          415a1da470f0a73906b6833dbf773cad 2010-09-25 1:14:57  jim         2010-09-25 1:45:14  null       null     null null    2     0       0         
459988 1          415a1da470f0a73906b6833dbf773cad 2010-09-27 21:58:45 Jim         2010-09-27 21:58:45 null       null     null null    1     0       0        

I don't know about you, but I think its safe to assume they are all the same person.

Any thoughts?

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That Jim user now has 130 accounts. –  M. Tibbits Jul 16 '11 at 12:31

2 Answers 2

Remember that users don't have to register to use the site. If Jim uses it on several different computers and/or browsers it could account for several cookie based accounts all using the same email address. If he clears his cookies occasionally, he may have created dozens of accounts over a period of just a few months, interacting with the site without actually creating an account.

A lot of users didn't like openid, and refused to use it in the beginning. He could even be one of those. Can't track their own questions easily, but at the speed questions are answered one doesn't really need to. Leave the window open, hit refresh after 5 minutes, and you have your answer, or turn on email notification for that question.

While stack overflow could associate accounts with the same email, this was considered unsafe early on due to the possibility of someone claiming someone else's account and/or questions and associated rep simply by typing their target's email in. OpenID is the method used to provide security, and having two separate primary keys would lead to problems, especially as user's email are expected to change as they change companies, etc.

So any account associations are done by hand. I don't know that there's a good reason to force association on people who clearly aren't interested in registering.

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4  
It does pollute the database somewhat though... –  Benjol Mar 17 '11 at 7:13
1  
@Benjol I suppose it does, but the effect in terms of users using the site, is minimal. –  Adam Davis Mar 17 '11 at 13:34
    
It also severely damages the system of reputability. Bad users can be noticed fast by the community if all of their questions are crap. But if its a bunch of crap questions over multiple accounts. It also skews statistics and other things. While its not a dire problem it is getting out of hand. Merging should be a semi-manual process, but most of the discovery needs to be automatic. Eg accounts sharing an email hash, username, and possibly IP. –  TheLQ Mar 18 '11 at 3:04
    
@TheLQ "it is getting out of hand" Is it? I would certainly appreciate some insight into how many of these accounts are from bad users that are able to hide it due to their access patterns. If it is as severe as you describe then we should certainly consider associating them automatically, but the few accounts you mentioned which I poked around in don't seem to demonstrate this. Perhaps you can find some good examples of this abuse? –  Adam Davis Mar 18 '11 at 3:13

If anything, that looks like either a bug in the data or a bug in SO. That isn't an account that's doing a lot of self promotion, since almost none of those duplicate accounts can even vote. It's a weird artifact.

Also, googling some of those turns up interesting things...

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