We have the tags add-in (78 questions) and python-addin (8 questions). Some ArcGIS Python add-in questions are marked with one or the other, and a few are marked with both.
Should python-addin be made a synonym of add-in?
I have suggested that it should.
To me the python-addin tag is redundant because we have the python tag, which enables Python syntax highlighting, and the add-in tag, which in the context of the python tag and an ArcGIS-specific tag such asarcgis-10.1 or arcmap indicates that we're talking about ArcGIS Python Add-Ins.
Also, ArcGIS Add-Ins can be written in other languages, such as .NET or Java. But no one has felt the need to create a language-specific tag for those.