Stupidly opened a 2nd credit card by Hudson's Bay. What risk is there to close it?

I kind of stupidly opened a second credit card (I went shopping and they said if I have a Hudson's Bay card, I can get discounts, and my dumbass went with it). Now I have learned that it's dumb to open more credit cards (especially since I don't intend to use this particular card much).

I have also learned that opening a credit card lowers your credit score. And closing it too?
I have had my main credit card since age 18, if relevant it has a $6000 credit limit. This second one has an $800 limit and has not been used. Oh, I am 22 right now, and in Canada.

If I were to close it, how much harm is there? It's 2-3 days old now....