Exact replica of small fruit stone (4)
I believe the answer is:
spit
'exact replica' is the definition.
(the spit of someone is their double)
'small fruit stone' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fruit stone' becomes 'pit' (I've seen this before).
's'+'pit'='SPIT'
'of' is the link.
(Other definitions for spit that I've seen before include "turning means of 8 [COOKING]" , "Thin metal spike to hold roasting meat" , "Rod used for roasting meat" , "Gob" , "cook a roast?" .)