You are trying to Add a Control and that Control already exists in your VisualTree. The solution is simple
You have Source Parent Container = The control in witch resides the element you want to add to the other container Target Parent Container = The control in witch you want to add your element.
To solve your problem just remove the element from the Source Parent Container and then add it to your Target Parent Container
Note: to have an answer more specific to your situation you must post your view or the lines where the error is getting thrown