Use the Task Usage view to check if a single resource is assigned full time to two or more tasks at the same time. One of ...

Use the Task Usage view to check if a single resource is assigned full time to two or more tasks at the same time. One of the tasks might be scheduled so that its dates cannot be changed. Try changing the constraint on one of the tasks to As Soon As Possible or, for projects that are scheduled from the project finish date, As Late As Possible.