After import from CF card, then next import with "don't import suspected duplicates'' checked, failed many times. You must close and reopen the import window, repeating the operation (one or two times) to make it work properly.
Agree with the above, I'm getting the same problem.
LR 4.2 is dead slow importing photos which is strange given that this was one of the few functions that was actually faster in 4.1 and it was fine in 4.2RC. I imported 50 shots from a Nikon D90 SD card last evening - it took 40 minutes and missed 2 shots which 'could not be read'. Re-importing picked them up OK.
Allegedly 4.2 has fixed the problems in 4.1; well, no. It's still dead slow, and now we have new problems that weren't there before.