As far as I know, the concept of readymades evolved from Dada ideas. Dada-ists used garbage, newspaper-clippings, Objets Trouvees as material to make art. The objet trouvee already contains the "pop" idea of using everyday materials - like newspapers - instead of more classical materials - like paint and canvas.
Duchamp added to this concept by using readymades, he also gave the idea a theoretical context.
Warhol added to this development by using readymade concepts. He took a well-known concept, not necessarily its material representative, and made it the subject of his works. I would say that Warhol used ideas and images in a similar way that Duchamp used objects.
In other words, Warhol or Duchamp may or may not be credited with something, but their "pointing and calling it art" is part of a development that didn't start or stop with their work.
Art-historians tend to talk about readymades as belonging to Duchamp. Warhol is credited for incorporating popular ideas and methods into art.