itye1970 explicitly asks for copying, because the depot has reached its maximum.
So in answer to that: no you can not copy lines. However there is no real need for this.
Firstly, you can assign more vehicles to a depot as its capacity. The capacity only shows how many vehicles can be inside the depot at once. If you for example have a line which need 50 vehicles all the time, simply buy 60 vehicles. 50 of them will be on the road all the time and 10 of them will be in the depot. When one vehicle comes back from duty another one will be dispatched and the returned one can be repaired.
If you really need more space for vehicles inside a depot, than you may consider adjusting the line length or the timetable. For example: if there are only 10 vehicles used at night but for peak hours you need 40 vehicles, you will have a surplus of 30 unused vehicles in the depot at night. This would cause the vehicles to jam in front of the depot. Consider adjusting the timetable in a way that the difference between peak and low hours stays reasonable, or make sure that vehicles waiting in front of a depot at night don't disturb the traffic or other lines by placing the depot separated from the main streets.
Building two separate lines following the same route is not recommended! Most likely, the pathfinding AI will mess things up with this configuration. Better split the route into two parts if necessary.