When something is samey, it keeps giving you the same kind of thing, so you get bored. A samey playlist is song after song that all feel alike. A samey town is street after street with the same shops. It is a casual, slightly complaining word — you would say it to a friend, not in a school essay. British speakers use it a lot; American speakers usually understand it, though they more often say repetitive. It does not mean “the same as something else.” It means “too little variety.” Though actually, people also stretch it to food that is fine but never surprises you, or to a week that was all right but had no change of flavour.