He's twice a father (4)
I believe the answer is:
papa
'he's' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'twice a father' is the wordplay.
'twice' indicates the doubling of some letters.
'a father' becomes 'pa' (pa can mean dad or father).
'pa' duplicated is 'PAPA'.
Can you help me to learn more?
(Other definitions for papa that I've seen before include "Pop" , "Father; letter P" , "Father; P (radio comms.)" , "Mama's other half?" , "Comms code for 'P'" .)