If your soulseek and music directories are on the same partition, you can do hardlinks (cp -l) instead of moving files (mv), so the same file is available from the 2 locations while its content is only recorded once on your filesystem.

Can't you just search whatever files you're sharing to see if you have it?

Assuming your filenames are unique, you can just run a recursive search for the file on your whole music library:

find /path/to/search -type f -name "filename.ext"

I use beets.io to manage my music. Once you've got it set up, you can just search for stuff with beet ls

I would keep track of them in Tellico, it is designed for this purpose.

Could you just set soul seek to the same directory you're moving the files to eventually and save a step?

I use sticky notes on my monitor.

Can't you just use fsearch on all drives?

If you use that to download straight to the folder you want to keep the files in, windows will tell you if you are attempting to download something that is already in the folder.