Applying the above method from bigjohnsta resolved it, thanks!
How I "created" my duplicates in the first place:
Switching from iPad 4 to iPad Pro, I initialized my iPad Pro with a backup from the iPad 4.
The iPad 4 was set to sync all music with iTunes.
Since the Mac that runs my iTunes was not available, I downloaded my music library via the iTunes Store.
When I later connected the iPad Pro to the Mac, music was finally synced... in a way that I had many songs duplicated.
Ok, so this time you could say I asked for it. Still, I think Apple should figure this out.
One minor issue, but not really increasing confidence in iTunes:
During the last step (Re-Sync), suddenly all my Playlists on the Mac had been duplicated, like this:
...
Classics
Classics 1
Contemporary
Contemporary 1
Cumbia
Cumbia 1
...
I had to manually delete all the duplicated lists.
Syncinc all the songs via the store and later via my Mac took hours, both in terms of download time and my time to figure out the issue. Also, once again I got the message that some objects could not be synced, had this issue resolved to have no "broken" unsyncable objects anymore only recently. Duplicates seem to happen a lot (and also songs suddenly missing, or unsyncable). I had several calls with Apple Support about this during the last years.
It is very difficult to believe iTunes developers know what they are doing. Personally, I don't think they do.
