The way I think of it, there are four distinct variants on "duplicate cards," all of which are described above:
1. Duplicate basic effects. I adore Firebolt and Burst Lightning, and basically no other 1-mana Red burn spells (
Gut Shot,
Galvanic Blast,
Lava Dart,
Lightning Axe--you guys are okay too dw), so instead of running a bunch of second-tier utility spells that would be otherwise different simply for the sake of being different, just run multiples of the same card.
Corollary: If I see you running
Thraben Inspector and
Novice Inspector in the same cube, I will physically take your Novice Inspector out of the sleeve and put one of my seventeen pocket Thraben Inspectors back in the sleeve in its place. It is the same goddamn card. If you want two, just run two!
2. Duplicates of build-arounds. This is your classic
Wildfire and
Burning of Xinye type of deal. The trick here is to make sure that only one person gets both copies, which is sometimes easier said than done. Don't ask me what happens when they wind up in the same pack. This one is the version that is best solved by running tutors or vouchers. Pro tip: make a custom tutor card that only searches up foil cards and then carefully select which cards are foiled. (I'm fascinated that this hasn't been an Un-card yet!) Or you could just put a sticker on the inner sleeve.
3. Color balance. Maybe you're running a 540 and don't want the GU drafter to get randomly screwed out of seeing Breeding Pool, so you add a second or third copy to make the odds of them seeing it way better. Most people are fine with this.
4. Multiple people will want a copy or copies of a given card. As in #3, several drafters could want precisely one Breeding Pool, but be planning to fetch it off of separate shocks because one is BGU and the other is RGu. This is a fairly land-specific thing, but it can happen with some specific effects, especially combo pieces that work for multiple folks--I'm thinking of stuff like
Emrakul, the Aeons Torn which can be both a cheat target and part of a re-shuffle package.
So why am I taxonomizing this? Mostly because people tend to want some but not all of these things. For example, I find 1 to be a really compelling reason to break singleton, but not so much 2, because I think that it's fine if games open the same way but I don't want them all ending the same way.
...
As for reeling it in, the way I tend to choose to duplicate a card is during the initial editing phase. I'll build the cube singleton to start, then I'll see some card and think that it would be better if it were a second copy of XYZ, so I just do that. I tend to not go to three unless I want the cube to start building around that card, to be honest, but it's a really good way to set expectations such as "Red does 2 damage for one mana and 3 damage for 2" or "UU means a hard counter but 1U will be a soft counter of some kind," especially when satisfactory versions of that effects are highly limited.