Plagiarist, imitator (7)
I believe the answer is:
copycat
I believe this is a double definition.
'plagiarist' is the first definition.
(I've seen this before)
'imitator' is the second definition.
(synonyms)
(Other definitions for copycat that I've seen before include "Emulator" , "One imitating another" , "Highly imitative" , "Done in imitation" , "Person (or crime) imitating another" .)