Someone who gives their Stamp of Approval to something after someone else has already recommended it.
Originally from the TV show How I Met Your Mother Episode: The Stamp Tramp
Person 1: I just really like the new song by Fun.
Person 2: I have not heard it yet (than goes and listens to the song on YouTube)
The Next Day: Person 2: Have you heard that new song by FUN its one of my most favorite songs of all time.
Person 1: I just told you about that song yesterday. (Rolls Eyes) You are such a piggyback stamper.