I received MP3 of Vocals and Bass recording, the studio artist said he sent it over as a Stereo file (not L/R mono). But when I import the MP3s into Ableton I get two duplicates of the sound wave in one track. Essentially one on top of the other. And I can't seem to remove one of them or isolate one. Has anyone ever encountered this before?

That's normal, an interleaved stereo audio file shows up as two waves in one track, for the left & right sides of the stereo audio -- two linked channels in one stereo file. If what you actually need is separate vocal and bass parts, unless he isolated the vocal on one side and the bass on the other the best thing to do would be to ask the artist for two different audio files.