You need to delete the named buffer after you have used it:
Code:
(gimp-edit-named-copy drw "tobepastedleft")
(set! floatingLayer (car (gimp-edit-named-paste leftlayer "tobepastedleft" TRUE)))
(gimp-floating-sel-anchor floatingLayer)
(gimp-buffer-delete "tobepastedleft")
(gimp-displays-flush)