Letter opener for post – share with another (6,2)

I believe the answer is:

double up

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'share with another' is the definition.
I can't tell whether this defines the answer.

'letter opener for post' is the wordplay.
'letter' becomes 'doubleu' (I've seen this before**).
'opener for' says to take the initial letters.
The initial letter of 'post' is 'p'.
'doubleu'+'p'='DOUBLE UP'

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(Other definitions for double up that I've seen before include "sleep together" , "Bend forward in pain or laughter" , "Look pained" , "React to punch in gut" , "win?" .)

I've seen this clue in The Telegraph.

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