Kate Middleton: Princess Diana Copycat

Kate Middleton will be the last person to complain about hand-me-downs from her mother-in-law -- she stepped out in a Princess Diana classic ... her fav pearl and diamond tiara.

The tiara's a big deal -- it's called the Lover's Knot Tiara and Queen Elizabeth gave it to Di on her wedding day in 1981.

Kate wore the headpiece Tuesday to the Diplomatic Reception at Buckingham Palace ... the first time anyone's seen it in public since Diana's death.