Kia ora! Welcome!
Some clarification (that you may already know): You don’t ‘upload’ anything into MusicBrainz, which is a music information database. However you might add new releases/information.
MusicBrainz is a music database. There are applications that use its data to tag/organise your music: the main one is MusicBrainz Picard.
This is definitely a common question! The easiest way is simply:
Picard will name files that are duplicates with a number on the end, eg:
01 Coffins.flac
01 Coffins (1).flac
Then you can search your whole music folder for ‘(1)’ (and (2) etc if necessary), and delete them.
Note: there may be songs that actually have (1) in the title. Make sure you’re only deleting duplicates.
As mentioned earlier, definitely back up your collection, or work with a small copy of albums/songs first. Changes to your tags or filenames can not be undone.
Then have a play! All you really need is the Picard quick-start guide:
You’ll probably have more specific questions once you’ve started, just post them in here.