One dip does not guarantee nudibranch eradication, never has. Inspection after a dip is just as important as the dip. Lets say you dipped the rock 5 times before you placed it back in your tank, yes, you can still have nudis if there are sticky egg sacks attached to the rock. You have to do a detailed close inspection of the rock with an eye piece and tweezers under actinic lighting if possible. The actinics will highlight the white egg sacks. Again, even after a dip, and once incubation periods end, whammo, you can get nudis.
I think the very best thing to do to insure that no host or predators of any kind will enter your tank, is to dip, inpect, quarantine for 2 weeks, then dip again before placing it in your tank. Just my 2 cents.
Mooch