Will Add(myctrl) add it to the window only once regardless of how many times I call it?
Uhm, depend on how you define "add it". The effect is more or less same as remove/add pair (except details). "myctrl" is just one entity, it cannot be somewhere "twice". Add has inside logic that makes sure it is properly removed from previous location.
Note: You can even Add(myctrl) to other window, effectively moving it from one window to other.