I've noticed that almost every posted deck has a single copy of unique characters. What is the reasoning behind this?

Too many unique multiples can be a liability, because if they die before you draw the copy it is a dead card. Different characters also give you more bodies to put on the table. For example say you have 2 characters on the table, and draw into a copy of one of the characters already in play. Then you still only have 2 characters, as opposed to drawing a different character who would give you a 3rd attacker/defender. So for many decks you will only see multiples of really key unique characters.

I agree. I only run more than one of a unique card if it is really worth the added risk (e.g. Robert Baratheon).

I will be honest with you, I run alot of duplicates... Depends on the deck and what you are trying to do. Baratheon Power rush, multiple duplicates. Martell 3x copy of the red viper. etc...