You can find multiple summoning points for each boss in a world but bosses are singular. You summon one and then go after it. If you find a vegvisir it marks the closest summoning location for that boss. You can summon the boss from each location but you don't need to there's no special reward for killing the boss at each possible summoning spots. You can kill the same boss multiple times if you want the extra trophies. You can have one on the stones and one on an item stand in your base that way.