This problem presents itself only if a user has two open (unfulfilled) assignments, both with the same recipient.
1. Get two challenge assignments on two different challenges with the same recipient.
2. Select the button to fulfill one of them.
The form will have both assignments checked (as if the work were fulfilling both), and both collections added.
Unchecking one of the assignments allows the user to post properly fulfilling only one of the assignments. (Provided they notice.)
If the user doesn't notice the second checked box and posts the work fulfilling both assignments, the second assignment can be removed from the work by editing the work (edit the work, uncheck the unwanted assignment, remove the unwanted collection, post with or without preview).
As long as the second assignment stays open, the form defaults to checking both whenever the work is edited, forcing the user to keep unchecking the second assignment.
The problem stops once there are posted works for both assignments, and posting the second work doesn't present the problem at all (because the first assignment is already fulfilled at the time).