Gilgulim?

Rashi in last week's parsha (כי תבא) quotes a gemara that seems to contradict the whole idea of gilgulim. The pasuk states ברוך אתה בבואך ברוך אתה בצאתך. The Gemara in Bava Metziah (107a) comments on this pasuk that it is teaching us that just like a person enters the world without sin he should leave the world without sin. The רש"ש there comments that this contradicts giglulim. The reason being, that the premise of a gilgul is that a nefesh that already sinned comes into the world to be מתקן that sin. However, the gemara states explicitly that a person comes into the world without sin.