Sometimes they borrow ideas. I recall a while ago I made a champion (Years ago, it probably isn't there anymore) who used a massive anchor to swing and beat people down. About a year or so later, Nautilus got released. Same weapon, entirely different kit, entirely different appearance.
Does this mean that Riot "stole my idea" for an anchor weapon? Not one bit. Archtypes and similarities exist for a reason. You're certainly not the only person who came up with, say, skeletal knight or medieval witch doctor. Does this mean Riot is ALWAYS original? Again, no (Though it's a poor choice of words on my part). Riot is always looking for inspiration and making champion concepts from some of the best the community has to offer.
Case and Point: 
was definitely inspired by angus, a joke concept made several years ago.
One thing to always keep in mind is that the MOMENT you make a concept, you're forfeiting rights. Theoretically if you made something so amazing that Riot 100% wanted it right stat now, they could use it without giving you a dime. It's one of those legal things to prevent someone from going "oh hey you stole my concept."
But this isn't about taking concepts, stealing money, whatever. If you make concepts because you want to make money, that's a terrible plan. You should make them to make yourself happy and gain experience in doing so. If your concept gets used? Awesome, add that to a resume when applying to be a champion designer or community lead. If not? Oh well, at least you made something unique.
Your goal should never be "I want Riot to use this". Your goal should be "I want to make something for the community that people enjoy."
