That 'sounding like they're singing back up' thing seems to be common in pop and hip hop music nowadays. I, too, was a bit surprised at these songs (I actually listened to them after I posted the link). What I'd heard before was her singing by herself over a kind of trance/techno music. A bit different from this. I suspect she also wrote, produced and arranged these as well as performance. She's a very capable and talented person.

Before they made me they broke the mold!
http://home.roadrunner.com/~kflach/