I just did a duo S&P E15 with someone. We had a sniper, but neither of us got sniped the entire time.
@Nirvelle
I should have mentioned this was in the Castle, no probs in the others.
Did you make sure it was an enemy sniper? Were you intercepting waves in places that had LoS to snipers? Second one happened several times to me, since Castle has a peculiar placement for snipers which sometimes means total security in main hall and second floor.