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