We have no evidence that Montresor has committed this crime before. When he speaks of the pile of bones that he had spoken of earlier, he refers to this utterance:
Three sides of this interior crypt were still ornamented in this manner [with piles of bones]. From the fourth [side] the bones had been thrown down, and lay promiscuously upon the earth, forming at one point a mound of some size.
The bones in the catacombs are of people in Montresor's family who died long ago. They are not the bones of people Montresor has murdered.
What Montresor is saying is that he has returned to the pile of bones thrown down in the middle of the floor. At this point, he has chained Fortunato to the wall. Montresor begins tossing the mound of bones on the floor aside in order to get to the stones and mortar he has hidden underneath. We realize that it was Montresor who removed these bones from the wall earlier. We perceive that he has planned his crime very carefully.