Under the FRC blueprint, yellow-carded players would be sidelined for the rest of the game and replaced by a substitute, with no such replacements allowed beyond three per team, while a player earning three yellows in a season would serve a two-match ban.
"The only negativity I have seen around has been around the yellow card proposal," Duffy told RTÉ Radio over the weekend. "That may need to be looked at. There's no point going to Congress with a proposal that is not going to be passed."
He added: "There is a genuine concern that the proposal around the yellow card may be too severe... it may have to be looked at again, but that's a matter for the committee. We're still at the stage of drafting motions."