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UFC superstar Nate Diaz has a message for Conor McGregor -- just because you WANT a rematch doesn't mean you're gonna get one.

Nate joined the guys on the "TMZ Sports" TV show (airing tonight on FS1) -- and responded to McGregor's vow that the two will face off again to avenge his loss at UFC 196.

"I'm sure he does want a rematch," Diaz said ... "I've wanted a lot of rematches too. Time will tell how things play out."

Diaz also doubles down on his attack on Justin Bieber -- saying, "F**k that fool" ... after Bieber insulted his fighting style on social media.

There's A LOT more in the full interview which airs tonight on "TMZ Sports" on FS1 at 9 PM PT, 12 AM ET.