Songs in album showing up twice (and only one is playable) - question and solution?

@Jengeno said:
One quick question not quite related, but still, with the Plex Dance, it would help a bit:
Is there any way of asking Plex to scan only specific folders?

Only per the command line interface
https://support.plex.tv/articles/201242707-plex-media-scanner-via-command-line/

The alternative seems to be to create one library for each letter of the alphabet and then scan the correct library when I make changes, but… I am not sure it is worth the hassle of setting it up.

That is a very bad idea, IMHO. It will make several features of Plex unusable.
For instance, this little thing will work so much better when all music is in one library. The automatic playlist generation only works within one library:

When in doubt, opt for lesser libraries, not more.
It will also fit better with the navigation concept of “type-first”, which will get implemented across all Plex clients.