Any card which is usually an above average draw if you draw the first copy of it, but is worse to draw a 2nd copy of, should probably be one-of. (Unless the first copy is AMAZING, in which case you are justified playing more).
For example, a unique location of decent but not extreme power level are usually good 1-ofs, like Chamber of the Painted Table. An extremely powerful unique location like The Red Keep is a 3-of, even though the 2nd and 3rd copies you draw are bad (except against Greyjoy).
Additionally, any card that can be a target of a tutor (deck search) effect, and is useful, should justify a 1-of. If you play support of the people, you should be playing 1-of locations that you might want to fetch with it. Of course if you find yourself never searching for a certain location in your games, its probably cuttable.
For characters, I actually find duplicates of larger unique characters to be INCREDIBLY useful. A significant amount of the decks in the meta are attempting to kill off your big characters in various ways: Stark, Targ, Greyjoy, and Varys decks (which is most Lanni, Tyrell, and Martell). Targ's burn kill goes through duplicates but everything else doesnt, and Targ still has trouble burning 5+ str characters a lot of the time.
Basically only Baratheon and Nights Watch make you not care about your duplicates of large characters. Therefore I tend to play 3 of any character in my deck that has 5 or more strength. I would usually rather have 3 of a certain large character (the more powerful one) than 1-2 of two different large characters.
For small characters, you need to expect that there is a reasonable chance they will die to military claim or wildfire. Therefore you should be much more careful including duplicates of them in your deck. If you do, maybe 2 is the right amount, not 3. This is why I only play 1 copy of The Ticker in my decks for example: he is better than a generic 2 str bicon, but not by enough to justify the risk of a dead draw if I draw a second when he is dead. If they play a kill effect on the tickler I don't really care, I am happy it didnt hit my big guy.
The idea that 'you don't draw 1-ofs enough so they dont matter' is very wrong. This is not magic. You will draw 12-14 or so cards before your first marshalling phase of the game, or over 25% of your deck, and more as the game goes along with 2 cards a turn draw plus numerous draw effects. You'll see your 1-of all the time. It will probably matter multiple games during a tournament.