While it is true there is no limit to the amount of Flip Summons a duelist can do in one turn, if you're talking about flipping summoning a card, then turning the card face-down and flip summoning it again, you cannot. This is because a monster can only change it's battle position once per turn unless it is through a card effect, and though it is true that Golem Sentry can change itself into face-down defense position through it's effect, you would not be able to flip-summon it again until the next turn, since there is no effect that would allow it to.
But, if you're saying that you have two Golem Sentry on the field in face-down defense position, then you can flip summon both of them.