I myself am a hardcore single-copy runner, but I would say no -- you're not doing anything wrong. This depends
greatly on exactly how you play. I'd advise most people that multiple copies of key cards is at least where to start and often where to end. Having your Free Peoples fully set up by site 4 (or even 6) easily sets the tone of the game and your ability to double past an opponent. Especially for classic strategies like Elf+Man Super Friends where you play a bunch of expensive permanents -- getting it all out while your opponent is roaming and can't react and then effortlessly sliding down the sitepath is a clear path to victory.
Now, that said, it shouldn't be as big of a surprise since decks have always had
some singles. One copy of
PATHS is an easy and versatile add, for example. For many decks a lone
Sam, Son of Hamfast is all the burden removal it has. I think once the core of a deck -- say, 23 cards -- is established, it can make sense to fill the other 7+ card slots with a single card that fills a particular need than with 4-5 cards to round out what the core should already be doing consistently. Depending on the format there are easy checks: for example, every Fellowship block deck needs "a Balrog contingency plan." There's also
KoI with 3 9+ strength Uruk-hai that you'll need to be able to survive. A 16-strength
Ulaire Attea, Keeper of Dol Guldur at site 3. Moria swarm.
Brown Lands bombs. Before my first game with a deck, I know that these are possibilities and I figure out at least a plan of what I'll do if I come across them (even if the plan is just "run"). Especially for something like
Brown Lands where I'll be able to see it coming from site 4.
For some of them the core deck already has it covered (
Armor does wonders for both The Balrog and
KoI, for example), but otherwise this is where single copies shine for me.
Betrayal of Isengard isn't ever going to be a core card, but tossing a copy into a Gandalf deck is nearly effortless. Now that might be a bad example because it's obviously written to be a splash card, but the idea stands for basically any card. I personally go way more esoteric --
Hobbit Appetite to ruin indirect wounding (5 wounds healed sets an opponent back an entire site), for example. The point is you rarely
want to see these cards twice, even in the games where they help.
I think another part of it is that some people are realizing that sometimes you don't need 30 cards when 25 will suffice. In the Elf+Man Super Friends example, taking it slow at the start and then smashing everything for the rest of the sitepath is pretty typical. A lot of people play this deck in a way that they get everything they need in the first 20 FP cards because of all the duplicates and doesn't even really
need the last 10. That's optimization waiting to happen. If they could stack the deck there'd probably be ~15 singles you get exactly when you need them and then the rest is events and non-unique permanents. That is of course impossible to make happen without cheating, I just mean to say it's not so crazy that there are some more decks on that side of the spectrum as everybody continues looking for that edge to win a few more games.
Multiple copies of cards increase consistency at the cost of diversity. And multiple copies of permanents (especially unique cards) increases stability at the start of the game while decreasing it later, except cards that meaningfully stack or that you plan on needing to replay. As I said, where you fall on the spectrum depends on how you want to play, how much risk you're willing to take, and what you want the deck to do. Decks with either a very consistent or a very fragile "core" are, in my experience, often well served by singles -- consistent decks have less need in typical games and more use for meta counters, while fragile ones have lots of need in typical games and need to cover a ton of ground. Decks in between are probably already covering their bases well -- they just need to draw what they have when they need it. Enter multiples
In case any new players are reading this, I want to be clear that this is all just for FP. For a Shadow side it's quite a bit different because only conditions or support-area items are permanents. You will see
some people (such as myself) diversifying their Shadow deck, but on the whole you can't go wrong with increased consistency there. Shadow gets to apply whatever kind of pressure they want and the Free Peoples bear the responsibility of handling it. That's probably another big reason to play singles: to stretch a deck to handle situations in the meta without just giving up and playing a different deck with a better advantage.