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(TMZ) - Months before he reportedly knocked up his baby mama Nia, Chris Brown was partying with her AND Karrueche Tran at the same time.

Karrueche, who was Chris' girlfriend at the time, actually had her arm around Nia in a photo they took -- with Christina Milian too -- during a Las Vegas party back in May 2013. Chris was nearby ... mean-mugging with his homies.

Check out the photo below:

S/O them They always #STAYTRU !! @msblairr #SexiiNia @christinamilian @karrueche @karizma22

A photo posted by Kid Red (@kidred) on

Nia allegedly got pregnant with Chris' baby later that summer. We're guessing the ladies won't be having a reunion anytime soon.

According to TMZ, Nia was very popular on the video vixen scene ... appearing in several music videos, including a half-naked role in Phill Wade's "Dreams of You" video.

Photo: Getty Images