"Overlaps this existing assignment."
You may encounter the message below when saving assignments. People assignments for a given project/task cannot overlap another assignment’s date range for that person/project/task combination.
Modify the assignment date ranges such that they do not overlap the existing assignment(s). Alternately, the existing assignment can be deleted or have its dates modified to avoid overlapping the new assignment.
If the project is set to "Manage at Task Level", another task can be chosen for the new assignment: when using a different task for the same person, the assignments can overlap.