To reproduce:

Add a RadCollapsiblePanel to a form. Add a button somewhere to the form. Continue working at design time. Copy the button, click the inner panel inside RadCollapsiblePanel and paste the button inside. Try to perform another paste or copy the pasted button, you will see errors.

Workaround:

Prior pasting click the header of RadCollapsiblePanel, this will select the whole control and perform the paste operation successfully.