It is possible one has a trailing space.
If you created User-defined genres, and any of them is the same as a default genre, they will be duplicated in the drop-down menu.
You can show only your own list:
I think I'm shipping the default installation with
Audiobook as an example for a custom genre. I'll look into it and either use a different example or remove any duplicates.
OP here. Thanks both ryerman and Florian for the helpful responses. I just checked and 'Audiobook' was listed as a user-defined genre. I've deleted it now, so there is no longer any duplication.
However, I know I never added 'Audiobook' as a user-defined genre myself, so what Florian said is likely the cause.
Have a great day.
It's fixed with Mp3tag v3.10a.