Duffy said: "I appreciate everything after [the life-threatening injury], to be honest. It's been six years since it happened and to be back in the fold is obviously a big thing for me and my family.
"But it's not just this, it's being back playing on the pitch and everything in my life, to be honest.
"I do look at everything a bit differently after that, to be honest. It took me a while to get going and I came back a bit early from it, but it's one of those things."
Derry-born Duffy was then an emerging player at Everton, but eventually had to take a step backwards before he could get his career heading in the right direction once again.
Loan spells with Burnley, Scunthorpe and Yeovil handed him the regular football he craved before he got his chance at Ewood Park, securing a permanent move in September 2014, and he has not looked back since.
His development has been such that he was worked his way into O'Neill's sights having been denied a debut under Giovanni Trapattoni by his injury back in 2010.
Duffy finally got his chance in an end-of-season friendly against Costa Rica in Philadelphia in June 2014, but had to wait until the 1-0 friendly victory over Switzerland in March this year for his second appearance.
However, he revealed he set himself a target as he watched his compatriots head for the Euro 2012 finals in Poland from his armchair, and that target could yet be achieved.
Duffy said: "I probably wasn't ready four years ago. I sat there and I was obviously watching the lads, and you want to go out and represent your country if you are in the game.
"It was just, 'I want to try my best to try to get to the next one - and if I can't get to that one, I'll try my best to get to the next one after that'.
"You have got to set it as a target and that's what I did, and hopefully I can be there."
Duffy's hopes of making the squad have been increased by the news that Stoke's Marc Wilson will not make it because of injury, while fellow defenders Paul McShane and Alex Pearce have been placed on stand-by.
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