If a player tries to take Foreign Aid and 2 or more players claim the Duke to block. Is this legal?
Or are challenges/blocks supposed to be done based on turn order?
We usually resolve challenges or blocks by whoever speaks up first. I suppose if it's truly simultaneous, then you can go by turn order.
In this case, if you want to challenge the blockers, you would first need to challenge the first blocker before challenging the second one.