Disclaimer: I'm not saying that the below is a well written Cordapp.
In 4.0, 4.1 and 4.2 it was possible to pass to CollectSignaturesFlow multiple sessions with the same party. The CollectSignaturesFlow would invoke every single session and collect signatures from it. Presumably resulting in the tx being signed by the same key multiple times?
In 4.3 the same usage results in exception: "There are multiple sessions initiated for party key"