However, such allegiances will be swiftly forgotten on Saturday when the League 1 Lions take on the top-flight Tigers.
"Matt and I have fought on the same side against testicular cancer and been through so much, but this weekend we'll definitely be lining up as opponents," said Harris.
"It'll be great to pit our wits against each other and hopefully I'll be the one celebrating when the final whistle blows."
Duke was diagnosed with testicular cancer 12 months ago but has since made a successful comeback for Hull and played a starring role in the last round by keeping a clean sheet against Newcastle United.
"Neil's a great lad but friendship won't be a factor that will come into play on Saturday," said Duke. "Both Millwall and Hull are having great seasons but I'm confident we can go one better by knocking Millwall out this weekend."
No matter which team you will be supporting this weekend get behind Everyman and visit www.everymancancer.org (opens in new tab) and make a donation for Harris or Duke.
Everyman is also pleased to announce that it will continue to be working alongside The FA as one of their charity partners during 2009.
Everyman's Keep Your Eye on the Ball campaign was established in 2000 and will continue to raise awareness of male cancers within the football community with this year's focus fortnight in April.
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