And then they brought in another Riker because, drama
The result of a transport incident gone awry, Riker #2 was an even smarmier version of sixth season Riker. His apparent purpose was to show us the Riker that could have been, had he not dumped Deanna back in the day to pursue a promotion. The whole thing wasn’t quite the good/evil doppelgänger trope, but it wasn’t really anything else, either.