Trace Adkins In Cruise Ship Brawl With Impersonator

Country singer Trace Adkins fought himself on a cruise ship Monday -- or at least he may have thought he was fighting himself -- because it seems Trace was really wasted and he just checked into rehab.

Sources tell TMZ ... Trace was headlining a country cruise headed to Jamaica. He was in the bar when something happened between him and a Trace impersonator who was doing Karaoke. Trace -- who had been sober for 12 years -- had fallen off the wagon and got into a physical fight with the impersonator.

We're told once the boat docked in Jamaica ... Trace jumped ship. We're told he's now in rehab.

Trace and the impersonator in question previously met in 2007 (see video below). The impersonator's wearing sunglasses.