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50 Cent 's screwing with Diddy a day before the mogul finds out whether he's headed off to prison or if he gets to go home ... telling the judge to think of the general public's need for baby oil.

The rapper shared the message on Instagram Thursday afternoon ... kicking it off by addressing Judge Arun Subramanian directly and mentioning his longstanding beef with Puffy.

50 claims he's feared for his life multiple times over the years, and he wants the judge to think about the "safety of the general public" when it comes to his impending sentence.

Diddy hasn't had enough time in prison to learn the error of his ways and truly reform, 50 explains ... regardless of teaching "Free Game with Diddy" -- a business class -- behind bars. Elizabeth Williams

Diddy's

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