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The original question title had been Why does the system allow adding duplicate or even multiple tags?

Just look at this questions tags to see the issue. I've been unable to reproduce this now both during initial posting and while trying to retag with a duplicate one.

Consequently the remaining (minor) question is why there are still duplicate tags once in a while, like for example this revision 2.

Presumably this could be caused by related tags having been merged into one, i.e. the renaming/merging of a tag-b into semantically identical tag-a on a question featuring both former tags yields a duplicate tag-a afterwards?

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3 Answers 3

up vote 3 down vote accepted

I've added a moderator function which deduplicates all current question tags, and all post history tag entries.

So the next time there's a duplicate tag problem, just flag one of them for moderator attention and we'll run the tag deduplicator page.

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Typically, if you try to add a duplicate tag, when you save, the duplicate is removed. However, in this case it appears it was part of a global retagging by a 10k+ user, note the change comment "win32-api -> winapi (latter has greater participation)". It must be that when this is done, the same duplicate check is not run. I would wager though, that if someone editted the question after that and did not touch the tags, the duplicate would be removed.

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Thanks for the explanation - the latter is probably the way things are, though this would classify as an after the fact solution to a problem best solved at its origin from a theoretical point of view, see my comment on TheTXIs answer. –  Steffen Opel Sep 8 '09 at 23:16
    
@Steffen, agreed it is as you say, likely a bug of low priority. –  Timothy Carter Sep 8 '09 at 23:18
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Just FYI, 10k+ rep users can't do this global retag business. That's a moderator action. But your explanation is probably correct. –  cletus Sep 9 '09 at 1:11

Sometimes tags are merged and get overwritten to be something else. So if you have something like

"C#" and "C-Sharp" and C-Sharp is merged, you will end up with two instances of "C#" on the question.

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That's what I've suspected already. Still this would classify as a bug, wouldn't it? Admittedly an extremely minor one, but the engine could and should take care of that - and I'm not even starting to challenge the referential integrity of the SO database schema here, which shouldn't allow for something like this to happen in the first place ;) –  Steffen Opel Sep 8 '09 at 23:12

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