I had an argument in a vs game with another player and this topic came up in a game and I figured i would ask.

What counts as a double pass in effect resolution?

For example:
Is this a double pass?
Me: I declare an attack on .... Pass.
Opponent:It's legal...pass
Me: pass
Opponent: pass

or is it as my friend said as this:

Me: I declare an attack on ... pass.
Opponent:It's legal...pass
Me: pass
Opponent: pass
Me: pass
Opponent: pass

The reason I ask is because this came up in a game where I pass on legality, and passed on effects, asked him if he passed and he said yes, than I said, sure, destroyer into shape is legal, you take 3, I take nothing. And then he likes, what? Realizes the mathematical error and says, what I want to add so and so to the chain.

I passed out while trying to make sense of that.

for an attack to become legal, it would go like this:

YOU - i declare my attack... pass
OPPONENT - pass

*BAM!* the attack is legal.

Of your two examples, the first is the correct way to go.

You declare an attack, ask if it is legal.
Opponent may play an effect that would dissallow the attack (exhaust your guy somehow, etc), or he may pass and say "Yes, it is legal".

Once the attack is legal, you get the opportunity to play effects. If you chose to play one, your opponent can respond to it, and thus you could respond to his. If you chose not to play effects, you would "pass".

The opponent now has an opportunity to play effects, if he chooses not to, he passes also.

At this point NOTHING ELSE CAN BE DONE about the combat that just happened. Compare your ATK and DEF values, and resolve the combat.