Sisqo -- Back On Thong Patrol ... Bigfoots Mini Sisqo

It must have looked like someone left Sisqo in the dryer too long -- until the 90s R&B star made a surprise appearance to do a duet with his little doppelgänger at Beacher's Madhouse.

At first, just Mini Sisqo got the crowd jumpin' Saturday night in Las Vegas with "Thong Song" -- and then out of nowhere the real deal singer jumped on stage and took over mic duties. The little guy managed to hold his own though ... while rocking a throwback headband. #respect

Turns out Mini Sisqo is multi-talented ... he also does a mean Mini Kanye.