Galaxy Watch

Transferring MP3s to the watch from the PC

You need AnExplorer to transfer the files from the PC to a website using the upload button.

Then you need MyWear File Explorer to select them all from the AnExplorer upload directory and move them to the Music directory

Then you need to reboot the watch for the music indexing to start.

Or you can use adb over wifi

adb push /path to music/ /sdcard/Music

adb shell "find /storage/emulated/0/ -exec am broadcast \
   -a android.intent.action.MEDIA_SCANNER_SCAN_FILE \
  -d file://{} \\;"

OR

an app like https://play.google.com/store/apps/details?id=com.kotorimura.refreshmediastore or sideload https://turndapage.com/navexplorer/

Transferring MP3s to the watch from the PC to the Phone to the Watch

In the wear app, go to Settings -> Manage content -> Tracks on watch -> Plus sign top right -> You can select "Folders" if you scroll the Playlists / Tracks / Albums / Artists bar!

Duplicate files in playlist

I had that issue I removed mine by going into Samsung files app. hit analyze and it shows what files are duplicated files. audio included you may have to check them off one by one as they are listed underneath one another. it is a bit time consuming but it does indeed remove the duplicate files form Samsung music. just remember to clear the trash can afterwards. each section in Samsung files has it's own trash bin

https://r1.community.samsung.com/t5/galaxy-s/samsung-music-keeps-adding-duplicate-songs-to-playlists-samsung/td-p/3636071