A case of identical twins! (3)
I believe the answer is:
pod
'a case' is the definition.
(pod is a kind of case)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pod that I've seen before include "Elongated seed case" , "Hull" , "A seedcase; some whales" , "A pea grows in it" , "Peas grow in one" .)