No, absolutely not.
The news piece that concerns specifically the Link's Awakening preorder explicitly clarifies that double gold coins apply to both a regular purchase as well as a voucher trade-in. In case of a voucher you only get half of the double gold coins (so 300 gold coins instead of 600 for Euro and US dollar currencies) because you already got gold coins when you paid for your vouchers.
In other news, sethnintendo needs to purchase a Switch because he is utterly clueless how gold coins work nowadays. Gold coins are cashback that can be applied to virtually any purchase on the eShop and that's without a doubt superior to having to pick rewards from a limited list. This is the best application of a reward program that Nintendo has ever had. Anyone who thinks otherwise after knowing how it works is plain and simply an idiot.
Really? Well, that's kinda cool and it seems I got it completely wrong.
So, you get the 495 from the vouchers, plus 300 from Link's Awakening. Making a total of 795. Or 7,95 euros. Sweet... you end up saving 28 euro.