Fans of Patty Duke are set to see double once more - the actress has signed up to play a pair of twin grandmas on hit Disney show Liv And Maddie.

The veteran entertainer will make a guest appearance alongside show star Dove Cameron, who plays the titular twin sisters, on an upcoming episode of the teen sitcom, set to air next year (15).

The 67 year old famously played "identical cousins" on her namesake series The Patty Duke Show in the early 1960s and Cameron reveals she cried when she learned her TV heroine would be joining the cast, because she studied Duke's double acting skills in preparation for the Disney Channel show.

She tells the Associated Press, "There's really only one model for this show. I didn't want to mess it up! I didn't want to offend her...

"They say you should never meet your heroes, but she's gone from a hero to a really, really great friend and mentor."

Duke adds, "It is an awesome experience."