Most people apply for a new B1 / B2 visa before their current B1 / B2 visa expires. Usually, if you already have one, the officer in the consulate will ask if you currently hold a visa (valid or expired), verify if it is the same one, cancel it & then issue the new visa. If you have a visa of different type (L1 / B1), it will be cancelled only if it is expired. Otherwise, they will usually let it be while they issue a new B1 / B2 visa.
However, if you happen to hold an I-94 based on the current visa that you did not surrender at the border when entering Canada, I remember someone telling me that could be a problem. I'm not very sure... but you better surrender that I-94, before you get your new B1 / B2 visa stamped.