NOT THE FIRST BABY CLAIM FOR BIEBER’S ACCUSER

The woman who claims Justin Bieber fathered her child -- originally accused her ex-boyfriend of being the daddy to the very same baby last year ... this according to her ex-BF's grandmother.

TMZ spoke with Frances Lippe ... who tells us Mariah Yeater had been dating her grandson John Terranova in Las Vegas toward the end of high school.