Back on? Katy Perry and John Mayer spotted together

She just can't quit him.

It appears Katy Perry and John Mayer are on again ...again. The "Who You Love" singers were spotted over Fourth of July weekend at the second show of the Grateful Dead's three-night "Fare Thee Well" concert series in Chicago.

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Perry, 30, dressed the part, wearing a tie-dye tank-top and ripped jeans. Mayer, 37, wore a blue bandanna in his hair, a denim shirt and loaded up on the accessories.

The "Firework" singer Instagrammed a pic from the concert, simply writing: "grateful."

The on-again, off-again couple also seemed more than willing to snap a few pics with fans, and they weren't the only famous faces at the July 4th show.

Woody Harrelson, Chloe Sevigny, "Game of Thrones" author George R.R. Martin and Jenny Lewis were all in attendance.

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However, Mayer might be the biggest Deadhead of them all. Billboard reports that the singer/songwriter has attended a total of five Grateful Dead concerts during their "Fare Thee Well" tour.