Buying one stick to match the one you already have is technically mismatching. Unless you get extremely lucky and end up with another stick from the same batch as the first, there will probably be some slight differences.

A lot of what you see online says that this is bad, and you should buy both sticks as a set, but realistically, you won't see a difference.

I've been doing it for decades, and never seen an issue, and that's in gaming and photo and video work :)