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Let's suppose a user posted a question about Javascript, he entered js and javascript as tags. But later, a moderator merged js and javascript into one master tag, it's now just javascript. Well, the system could simply execute an SQL update to replace the TagId of former js into the TagId javascript in a Tag Dictionary table, but the problem is, the post will finally have two identical tags (Looks like this Tags: javascript, javascript), so I was wondering how does SO handle this problem?

I know we could do a big loop for that update, but when a heavy website has tons of posts, the process will be extremely expensive and too long to respond.

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I'm not sure how useful it is to go into detail, but there is a de-duplication process here... – Marc Gravell Nov 20 '10 at 1:54

If I remember correctly, SO actually doesn't handle this, and the exact problem you described will happen: the post will have two identical tags

Edit: I tried it to make sure (I made the tag on a question that had and then merged the former into the latter):

Screenshot of the problem

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