The Hero you receive in a summon is selected randomly based on different probabilities depending on the rarity of the Hero. All players have equal chances for summoning, so while we acknowledge that getting a specific Hero is definitely not easy, we also consider the system to be fair.
With that said, it is indeed possible to receive the same Heroes (or duplicates) from the summons. Please note that you can use the duplicates to level up your other Heroes. The chance to power up a Hero's Special Skill will also be much higher when levelling up with the same Hero.