When you're affected by a crowd control effect in PvP, there's an 18-second window for diminishing returns on further effects from the same category. Additional CC effects used on you while diminishing returns are active are reduced in duration: first by 50%, and then by 75%. Each CC effect resets the 18-second window to its full duration, and if you are affected by three CC's of the same category inside that 18-second window, you will be immune to further effects from that category until the 18-second window has passed.
The lists below have been separated out into their specific diminishing returns categories. Each ability shares diminishing returns with the other abilities it has been listed with, and no others, except where noted.Diminishing Returns CategoriesDisorientIncapacitateRootSilenceStunKnockback