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Lil Wayne's infamously NOT released "Tha Carter V" has finally been released -- at least partially -- by who else, but Martin Shkreli ... and we've learned Wayne's pissed.

Shkreli pulled a sort of hip hop Robin Hood move, he claims, by posting this video of himself hyping up, and then playing a track he says is from Wayne's legendary album. He boasts that the song is about him.

Of course, Weezy fans have been dying to hear anything from the album since October 2014, when it was originally supposed to be released. Wayne and Birdman have been in a standoff and legal war over the record ever since then.

A source close to Wayne says this is the real McCoy. We suspect Shkreli's about to have even more legal trouble -- in the form of a lawsuit from Cash Money or Universal Music Group.

A source close to Wayne tells TMZ ... his camp is doing an internal investigation to find out if someone passed the song to Shkreli. As we've reported, Birdman says he doesn't have a copy of "Tha Carter V" ... and insists it's been in Weezy's hands all along.