One's twice becoming a goddess (4)
I believe the answer is:
isis
'a goddess' is the definition.
(Egyptian god)
'one's twice' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'twice' shows that letters should be duplicated.
'is' doubled is 'ISIS'.
'becoming' acts as a link.
(Other definitions for isis that I've seen before include "where one might take a punt" , "Egyptian deity" , "Oxford's second crew" , "God" , "Jihadists" .)