Hi, I would like to contribute to this problem.
I was still getting the same behavior that slots were set twice using the FormPolicy. This is especially inconvenient when having a validate_{slot} function with an utterance dispatched because then, the utterance is dispatched two times.
I tried to fix the issue by setting the slot in question to auto_fill: false. Now the Slot is not getting set twice anymore, but when I try to use the slot in my answer that is dispatched in submit(), I get a None printed out for this slot.
Maybe this is the same problem I am describing here?
The problem can be solved by getting the slot values in submit() and forwarding them to the utterance template as different named placeholders.