Twosome taking one month, repeat, one month interval (9)

I believe the answer is:

duodecimo

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'one month interval' is the definition.
The definition suggests a singular noun which matches the answer.

'twosome taking one month repeat' is the wordplay.
'twosome' becomes 'duo' (synonyms).
'taking' is a charade indicator (letters next to each other).
'one month' becomes 'Dec' (abbreviation for December).
'repeat' becomes 'imo' (I can't justify this - if you can you should give a lot more credence to this answer).
'duo'+'dec'+'imo'='DUODECIMO'

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(Other definitions for duodecimo that I've seen before include "small book" , "Size of book" , "scale of work?" .)

I've seen this clue in the Sydney Morning Herald.

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