Rafi Magid asked:

rashi explains a case that a person KIDESH 2 sisters. the first one with "kidushey safek" and the second with "kidushey vaday". why is it that we are CHOSHESHIM for a sin [one man marrying two sisters] like that?

The Kollel replies:

Since the Kidushin of the first sister was a Safek, perhaps the man will think to himself that it does not take effect at all (he will reason that she has a Chezkas Penuyah), and thus he will assume that it is permitted to marry her sister.

Mordecai Kornfeld