CALL'EM OUT! DAY 26 ACCUSED OF STEALING SOMEONE ELSE'S SONG!

P. Diddy handpicked Day 26 in Season 4 of "Making the Band" -- and now they're being accused of handpicking one of their new songs straight from someone else's album!
In a copyright infringement notice fired off to Bad Boy Records -- the company ran and founded by Diddy -- singer Kirby Shields alleges his song "I'm Just Getting Started" is "identical" to Day 26's "Just Getting Started," released in April.

And here's the rub: Shields says the copycat track was produced by Christian Davis Stalnecker, the same producer he hired to work "exclusively" for him back in '07.