Madonna fans should keep a close watch on the "MDNA" tour kickoff on May 31 for the setlist -- the veteran pop star may include a mash-up of one of her tunes with one of Lady Gaga's.

In a rehearsal footage YouTube video posted over the weekend, Madge is shown performing "Express Yourself," which then segues into Mother Monster's "Born This Way." The latter was often criticized for sounding too much like the former, and obviously Madonna was listening.

"I thought [when I heard 'Born This Way'], this is a wonderful way to redo my song ['Express Yourself']," the singer told Newsweek back in January. " I thought it was...interesting."

How interesting? Maybe Madonna thought it was a little too carbon-copied, because she ends this little song experiment with the refrain from "She's Not Me" (from "Hard Candy"). Is it a joke, or is it beef?

Check out a side-by-side comparison of the tunes below, as well. Here are Madonna's tour dates. Lady Gaga is on tour overseas currently in support of the "Born This Way" album.