kurwa is not used to end a sentence.
True, but it's often said after the briefest of pauses.
using a double kurwa in the middle of a sentence is acceptable.
Can't remember hearing a double K, but a I did hear a woman yelling at her husband, who was on his way back from the village shop after a few breakfast beers. After letting him have it she ended with, "Kurwa bardzo." Never heard that one before or since. Sounded almost polite.