i'm sorry to ask again,but i didn't get the answer,i already have a multiple visa,why do i have to apply for another one? isn't there anything as extending a multiple visitor visa? if not,then what's the use of getting a multiple visa instead of single? canada immigration system is soooo weird

i'm really confused,do you know where i can get answers to my questions? there is a phone number in the website but that's useless, i really need to be sure of the answers i get,unless it's a big risk for me:(