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On this post at Math.SE, one sees that it is tagged both with (multivariable-calculus) and (vector-calculus). If one clicks on the (vector-calculus) tag, one gets redirected, correctly, to the page for (multivariable-calculus), as for a long time the (vector-calculus) tag has been a synonym for (multivariable-calculus).

Problem: why does the post still contain both tags, instead of them collapsed into one?

What caused this:

  1. Mark and merge (tag-2) as synonym to (tag-1).
  2. Given a question tagged with (tag-1) and (tag-3), merge (tag-3) into (tag-2).

Ideal result:

  1. Question is now tagged as (tag-1) only. All questions previously tagged under (tag-3) is now tagged (tag-1) instead.

Observed result:

  1. Questions tagged with (tag-3) now is tagged with (tag-2), which displays as a different tag from (tag-1), but links to the same page.
  2. (tag-2) now appears in the Tool panel for "new tags" as a new-tag, even though the link that it creates points to (tag-1).
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I think it would be better to prevent merging into a synonym - you should always want to merge a "bad" tag into a "good" one, correct? – Jarrod Dixon Apr 10 '12 at 20:16
@Jarrod: yes. If that can be done, I would appreciate it. – Willie Wong Apr 11 '12 at 8:25
up vote 2 down vote accepted

If you make a synonym of , you are not removing from questions. Future questions using will automatically have , but that doesn't change already asked questions.
and are still two separate tags, if you make them synonym, and there are already questions using .

To obtain the result you wanted to obtain, you should:

In your case, you just need to merge with .

share|improve this answer
Right, but if I merge tag-3 into tag-2, isn't it just like removing tag-3 from questions, putting tag-2 to those questions. And since this happens after tag-2 is already merged and synonym'd with tag-1, shouldn't the newly appended tag-2 be mapped automatically to tag-1? – Willie Wong Apr 10 '12 at 9:08
That happens only when tag-2 is manually added to questions after it has been made synonym of tag-1. Merging two tags is not considered manually adding tag-2. – kiamlaluno Apr 10 '12 at 9:21
Perhaps I should make this a feature request then? – Willie Wong Apr 10 '12 at 9:24
You could make it a feature request. Automatically merging two tags that have been made synonym is not something I would prefer, though. If they have not merged the first time, maybe there is a reason they have not been merged. Consider that between step 1, and step 2 could pass even months. I would prefer the merging is made manually. – kiamlaluno Apr 10 '12 at 9:32
Ah! No, no, no! I think you misunderstood me. In step 1 of my scenario the tags are merged and synonymed already! I edited the question to clarify. – Willie Wong Apr 10 '12 at 9:34
In other words, at the time tag-3 was merged into tag-2, tag-2 already does not exist except as a synonym for tag-1. – Willie Wong Apr 10 '12 at 9:35
You wrote "Mark (tag-2) as synonym to (tag-1)." That doesn't mean the tags have been merged. – kiamlaluno Apr 10 '12 at 9:35
Yes. I just realised that's what you meant with your answer. So I edited to clarify. Sorry about the confusion. – Willie Wong Apr 10 '12 at 9:36

This is by design: merging into a tag prevents that tag to be a non-master synonym. We don't want to create a whole graph of synonyms.

Regarding how to obtain the result you want, kiamlaluno's answer is the way to do it.

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