I assume the XP can only be doubled, right? You don't double it again if you equip a second PDZ among your team?

As Chad said, max is only 2.3x (I.e. 1 PDZ and 2 Bonnas). The main draw for getting 2 is so you can hit the 2.3x max without using 2 Bonnas as trainers.

So... If I already got one from luck chest I can still get another another one from 1% drop? What about the third???

Yes, you could still get the 1% drop, or you could get it in a luck chest again. The 1 drop limit does not apply to luck chest drops.

I guess I'll go and get another one to remove normal Bonna from exp squad and gain one free slot. The fight was easy with gego L100 and sayu capsules. So 2 PDZ is max exp capped right? What about using exp booster too?

Fix me if I'm wrong, a PDZ with one Bonna shall reach the cap 2.3, as the EXP modifiers (PDZ, Bonna) are multiplied to get the final result, 2(PDZ) x 1.15 (Bonna) = 2.3.

Yes, you're right, tested on 1-1, the EXP modifier got from 1PDZ + 1 Bonna is 2.15. OMG, I originally thought it's 2.3 with 1 PDZ and one Bonna, and run so may MZ7 , My experiences Fortunately, I have 2 PDZ, I'll use two PDZ to run MZ from now on