On Ask Ubuntu, in the retag dialog, I noticed:
Notice that [fontconfig] shows up as both a synonym and a regular tag (which somehow has 4 usages). Clicking through "fontconfig" shows it does seem to be an ordinary synonym.
As I recall, normally if you start typing the name of a synonym, only the synonym target is shown up (with the name of the synomym bolded/highlighted); the synonym shouldn't be displayed as a separate tag.
I presume the bug comes from "fontconfig" still having usages, even though according to What are tag synonyms? How do they work?, synonymized tags are supposed to always have 0 uses.