There's nothing wrong with going back to the well and offering variations on a theme. Hip-hop is a referential medium, and plenty of artists have built sustained, flourishing careers on the basis of a few ideas flipped in new ways. (EPMD's business-mindedness comes to mind.) But at a certain point, you gotta let it go. To choose a particularly stark example: LL Cool J turning, "They call me Jaws, my hat is like a shark's fin" from the classic "I'm Bad" into the hook for "Deepest Bluest (Shark's Fin)," one of the deeper valleys in LL's sine-wave career. You made it a hot line, then you made it a wack song.