Crossword Genius

Double first's ultimate achievement (4)

Ross

I believe the answer is:

twin

'double' is the definition.
(twin can mean someone's double)

'first's ultimate achievement' is the wordplay.
'ultimate' indicates one should take the final letters.
'achievement' becomes 'win' (a win is a victory or achievement).
The final letter of 'first' is 't'.
't'+'win'='TWIN'

(Other definitions for twin that I've seen before include "(Town) paired with another" , "One born with another" , "Lookalike" , "Castor or Pollux?" , "One of two duplicates" .)

I've seen this clue in The Guardian.
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