Apologies for the confusion. After an assignment’s deadline has been reached it closes another assignment opens (called the Late assignment) so that students who missed the deadline can submit their late assignments. Unfortunately, Moodle alerts all students to the fact that there is another assignment with a looming deadline. If you submitted to the original assignment (as you did), you can ignore the "late assignment".
We are aware that is confusing and next year we will be changing the way we operate and having just one assignment for early and late submissions.
Head of e-Learning.