Reordering tags sometimes duplicates the tag

### Expected
Should be able to drag-and-drop to reorder tags without duplication.

### Observed
When dragging-and-dropping to reorder tags, about 25% of the time it duplicated the tag when dropping it. The duplicate tag persisted after a refresh so I don't think it's just a display bug. It seemed to happen more often the first time I dragged-and-dropped a particular tag, but I haven't been able to consistently reproduce.

### Where did you see the bug
OSX/Firefox

Start by reproducing the tag drag-and-drop behavior in the OSX/Firefox environment described, then trace the tag reordering flow. Done means tags can be reordered without creating duplicates and the result remains correct after a refresh.

Written by the indexing model from the issue text.