The 28-year-old replaced Adam Yates in the Vale side for Tuesday's win over Bury in the Johnstone's Paint Trophy.
Duffy believes that it is vital for all players to be ready for action, even if they are on the bench or left out of the matchday squad.
He told Vale's official website: "We get on together off the field [with Yates], which is good, although if you want to put it one way we're rivals, we play for the same team but we're playing for the same position.
"That's football you can't give yourself 11 players you've got to have a good squad for when people are out of form or injuries and then it's up to the manager (Micky Adams) who he picks.
"You're paid to play football and when you don't get picked on a Saturday it's frustrating and disappointing but you've got to get behind the lads who are playing."
Vale face Carlisle United in League One on Saturday.