Now while I enjoy the occasional nonsensical joke (Two ducks are sitting in a bathtub, one says, "Pass the soap," so the other says, "What do I look like, a typewriter?"), that's not why I find the "difference between a duck" joke so funny. Now a joke isn't funny if you have to explain it, but so many people have been expressing their incredulity at it that I feel compelled to defend it (and hopefully clear myself of the charges of witlessness). I always thought it was a very unique kind of joke I haven't heard any quite like it.
First, I think the question itself is funny. Why? Because it just fails to make sense at the last moment. Any way you attempt to parse it you get thrown for a loop. I guess it's a matter of taste whether you enjoy that or not.
The first answer continues in that vein with the same singular/plural confusion: It would make sense if it was "One of its legs is the same as..." or "Both of its legs are the same." Pieces of it make sense but not the whole put together I think it's something like a linguistic equivalent of the Penrose triangle. Actually, I think it is the equivalent, because that's exactly the same sense of amusement I get.
The second answer is the same, but additionally you can have fun trying to visualize the ludicrous image of a duck banging its head together when it walks.
The last one is just funny because it's absurd. I can't explain, just try to imagine acting it out to somebody.
By now, apart from thinking the joke is unfunny, you're also thinking what a loser this guy is trying to explain a bad joke and making a fool of himself in the process, so I'll just shut up now. But not before telling you the one about... ...the Buddhist who walks up to a hot dog vendor and says, "Make me one with everything."
Oh, and when the guy hands him a hot dog, the Buddhist pays him and asks for his change. So the guy smiles and says, "Change comes from within."