JAY Z's NELSON MANDELA SHOUT OUT: IF Y;ALL GOT A DREAM CHASE THAT MUTHAF***ER DOWN.

Jay Z performed a show-stopping tribute to Nelson Mandela in L.A. -- and if it sounds familiar ... that's because he laid down the same exact tribute to Trayvon Martin months ago.

During the intro to his hit, "Young Forever" ... Jay told the Staples Center crowd last night, "We wanna dedicate this song to Nelson Mandela."

Jay's Mandela Tribute...

• Heartfelt
• Cheap Trick

The crowd sang along and put cell phones in the air -- but some critics are calling out Jigga because during his summer tour he frequently performed the song and dedicated it to Trayvon.

Look, it's not like the song is NOT appropriate for Mandela -- but is the sentiment cheapened if it's a rerun?