When I was young, about 30 years ago, I'd had no problem to have two policemen, two yellow astronauts or knights. Then I had some lack of Lego sets and didn't know what was happening on this field.
Now, when my son begins (or rather "has begun") his Lego adventure, I realized, that there are theme sets based on movies, stories etc. But there are sets containing the same character.
As the Lightning McQueen is in three sets, there is no reason (from my point of view) to repeat Sheriff in 5814 and 5816 sets. So my son has two Sheriffs.
Some years later he changed from Cars to Lego Ninjago. On every occasion (birthday, Christmas, ...) he was receiving Lego Ninjago sets. Now he has eight Zanes, three Coles, three Kais, two Jays.
The same is now, but in smaller scale, in Lego Chima.
As I was looking at the Lego StarWars theme this is even worse. Ok, I can agree to have 8 clone troopers, but for what reasons have three Darth Vaders?
In my opinion this is not a good marketing practice. If for someone the sets are expensive, he would buy one set and (perhaps) a Luke Skywalker minifigure. He can re-think whether to buy a new set with a character he already has.
If each set had a unique minifigure, I would be more likely to buy more sets, just to make a collection.
It's also not ok (for me) to have the same characters for example in Lego Heroica.
I know that sets are larger and smaller, and there are people who buy cheaper ones, and they also should have something interesting, but why then copy the same characters to larger ones?
Does it has some rational explanation?