Isn't it possible that quotes have simply been rediscovered many times?

There are billions of people on earth, and we all have witty things to say. And inspirational quotes don't carry much information, so many people are bound to say the same thing, especially if you consider the meaning instead of the literal formulation.

The same thing happens with jokes, especially puns. For example, try making a joke on someone's name. Even if you just made it up, there is a high chance that the person in question have already heard it.