Dead rubber is a term used in sporting parlance to describe a match in a series where the series result has already been decided by earlier matches. The dead rubber match therefore has no effect on the winner and loser of the series, other than the total number of matches won and lost.
Dead Duck: A person or thing that is beyond help, redemption, or hope
So in the context of the Dublin/Roscommon game and taking the above in to consideration then would think it would be a Dead Rubber.
Funnily enough though I happened to come across both in the one place last week in Kilkenny. Was in Jenkinstown Park for an aul dander and not only was there a decaying duck in the woods but there was also a Dead Rubber lying in the car park which got me wondering should it be renamed Jiggystown Park."
So the game is a dead rubber but Roscommon are a dead duck.