It doesn't seem to take into account the amount of traits the pawn has gained thus far, as the mod page says, and my starting pawn which started with 2 now has 5.

From what I've observed, pawns seem to get traits randomly when they have a developed immunity rather than on actually reaching the 100% and when loading your game while they're in this state they have a chance to gain a trait. This possibly leads to multiple traits from one developed immunity condition.

I went ahead and did some testing with only this mod enabled and returned with the same results.

Steps: give disease & grant immunity, save game & load multiple times (10% chance to gain a trait)

https://imgur.com/a/GimHYIG