The thing is... why would you hire a "celebrity" to do a duck voice in the first place? All the duck ever says is "AFLAC". Any out-of-work voice actor or deejay -- hell, any five-year-old -- could do that.
I always thought it was someone doing a really bad Gilbert Gottfried impression, not the man himself.
That said, I think I have never laughed harder in my life than when GG took the podium several years back at the Friar's Club roast of Hugh Hefner. At Beej's b-day gathering the subject of celebrity roasts came up, and I made this point & everyone who'd seen it agreed. Don't have time to look for it on YouTube today, but I'm sure it must be out there somewhere.