A user can be assigned multiple times into a sprint.
Not easy to reproduce but suddenly occurs on some customer and test instances. Basically, you will just notice that a user is assigned several times to a sprint.
A user can only be assigned to a sprint once.
A user is listed several times in a sprint assignment.
"CLICK HERE TO COPY ALL MEMBERS FROM THIS TEAM INTO THIS SPRINT" will clear the duplicates but the user has to reassign everything again.