Whenever you try to assign a route which overlaps the planned duration of another route for the same vehicle (active or pending) you will receive the following type of alert (example):
If you proceed, you will create a circumstance in which at some point in time the vehicle is supposed to be executing two routes simultaneously. As a result, of course, in at least one of those routes the execution will not follow the plan.
The recommendation is not to create such overlapping. To avoid it, you can remove one of the routes, change the planned times of the route you are trying to add or choose another vehicle.