When Yosef reveals himself to his brothers the medrash says that he "proved" his identity by showing them his mila. Yet, in last week's parsha (Miketz) the medrash on the Pasuk
ויאמר פרעה לכל מצרים לכו אל יוסף אשר יאמר לכם תעשוsays that Yosef forced all the מצרים to do a מילה and therefore if even the Egyptians had a מילה what proof was Yosef trying to bring by showing them his מילה?
There are a number of answers offered but I believe that this raises a more fundamental question, do we need to assume that medrashim are not contradictory especially when Rashi quotes both? I can't offer a definitive answer but I would suggest that the answer is no. The purpose of medrashim is not necessarily to tell us the pshat in the text but rather to bring out certain lessons and therefore even if the medrashim conflict the lessons are still valuable.