double reaction

[Nevvy1]

with 6 cards in hand, I was able to use the same diplomat reaction twice responding to 1 action. This is clearly a bug with the interpretation of reactions.

[Ingix]

No, that's actually correct. Once you are done with Diplomat the first time, you are down to 5 cards in hand (from your original 6), so still meet the conditions for Diplomat's Reaction to use it a second time.

The reason why you can use the same Diplomat a second time is that nobody except you can know if it is the same Diplomat!

For ease of use, the game has you reveal and then use Reactions one at a time, then allow you to use more Reactions if you have them. In this play model, no other player can generally know if you had 2 Diplomats originally, or you drew another one from the first Diplomat Reaction use or you only ever had one Diplomat at all.

And in order to not have Dominion turn into a detective game where a player tries to reconstruct the previous turns to "prove" that the other player could not have had 2 Diplomats, the rule is that for Reactions from hand there is just no limit to how often one can be used. Other triggered effects from publicly visible sources can only be used once, because it's clear for every player how many of it there are.

The "newer" style of Rections that play themselves as part of the Reaction process limit themselves because they move out of the hand. The "older" style that just reveal themselves sometimes limit themselves (the condition on Diplomat is at some time no longer going to be satisfied), while others can in theory uselessly be revealed again and again (Moating an attack 100 times is not any different from doing it once).