In Reply to: Doubling midi tracks posted by Lacidar on October 08, 1999 at 03:15:55:

When two identical tracks are played from the same synth with the same patch, they will play almost in perfect sync which will result in this type of unpredictable distorsion you are hearing. Flanger, phaser and chorus effects are all using this phenomenon.

To avoid this you can either mess around with the offset of the tracks, causing bigger or smaller delays, till it sounds good, or use a diffrent program for one of the tracks. I recomend the latter.