You want to keep the first value. That's more complicated.
[Edit:]
Action: Format value
Field: Genre
Format string: $regexp($reverse($regexp($regexp($reverse($regexp($regexp(%Genre%,'^(\s+;*|\s*;+)+|(\s+;*|\s*;+)+$|((?<=;)\s+)|(\s+(?=;))',),'^|$',';')),'(;[^;]+)(?=(\1|;.*?\1))',),'^;+|;+$',)),'(?<=;)',' ')
Replace Genre with any other tag and it will remove duplicates too.
This action "fixes" first the multiple value string (remove multiple spaces after ; separator).
It will add exactly one space after value separator [;].
Enjoy. 