A duet or group song can be performed as a solo song as long as the defined new dramatic context makes sense. 

Regulation 1 states: All songs must be presented in a clearly defined dramatic context

This regulation allows Learners to take a duet and realise it as a solo if it works and you can even take a group song, where all characters are realising the same or a like objective and create it in a defined context. 

They can sing the words of the other characters but the new dramatic context needs to work.

 If answering a question on plot etc, this has to be responded to within the context of the musical from which the song has been taken. . 

Please ensure the author's intend is still clear and that the piece enables the learners to meet the assessment criteria to the best of their ability, answer the knowledge questions and comply with the regulations.