5IVE used to beat each other up before they went on stage.
He told Now magazine: ''We watched back the videos of 5ive and they used to throw each other down the stairs and kick the s**t out of each other. Then they'd get on stage and do a gig.''
Duncan also admitted Blue wouldn't have survived as a band if they've had suffered similar fights behind the scenes.
He added: ''Our battles were more about ourselves and learning who we are as individuals.''
Both groups recently reformed for ITV2 series'The Big Reunion', although the 'One Love' hitmakers made some enemies on the show when they joined part way through having already got back together in 2011 for the Eurovision Song Contest.
Honeyz singer Celena Cherry recently said: ''I was really angry about it, quite honestly. I felt like Blue had already come out and they'd done their stuff.
''All of us hadn't done anything in over 10 years. We were completely back, new to it after 10 years.''