The duties vary with the category of goods as well as with the place of manufacture.
The taxes vary with the province from 5% in Alberta (GST only) to 17% (HST) in New Brunswick.
Both will also vary with the price of the goods.
Note that items sold with Free Shipping actually have the price of shipping added into the selling price. (And this gets really complicated when the item has Free Domestic Shipping, but added shipping for international sales.)
http://www.cbsa-asfc.gc.ca/import/postal-postale/menu-eng.html
If you are ordering from the USA, from a seller using the Global Shipping Program, the Canadian charges plus a $5 or so service fee will be noted on the listing and you pay before the item is shipped. There will be no further charges on your doorstep.
Postal imports valued over $20 are supposed to be assessed for duty and tax at the border, but CBSA tends to ignore this if the item is low value (under $100) or small. Otherwise you will be paying on delivery.
In either case,you will not be paying double.
Either the duty and tax is paid before it enters Canada or you pay when you receive it. Not both.