Is it possible to cancel the effects of Smaugs shadow card? Or is this part of the PlayercardshavenoeffectonSmaug text?
I believe you can cancel the shadow effect because the shadow card itself is not immune to player card effects.
Do you deal a shadow card when Smaug attacks Lake Town?
FFG has stated that part of the process of an attack that occurs outside the combat phase is dealing a shadow card, so yes, the attack would get a shadow card.
And finally, can there be more than one Smaug card in the game at the same time? Rules say that you have to discard the Smaug card if you get another version. What if you draw the same Smaug version? Imagine you are the first player, choose to put Smaug to your area instead of the staging area. Then you draw the same Smaug currently engaged with another player. Is his version of Smaug discarded or your just-drawn version?
There is only ever one Smaug card in play at a time. If you draw the same version you already have in play, you still replace it. In the example you cite, the other player's Smaug is discarded and the new version is put into play engaged with you.