The custom is that the Sandek the person who holds the boy at a bris can only be Sandek for one boy in the family. If so how was it possible for Yosef to be the Sunday for the son's of Mochir? We compare the Sundek to bringing the ketores in the Bes Hamikdosh. The high priest could bring the ketores as many times as he wanted likewise the head of a community will be compared to the high priest and he too can bring the ketores in the same family on more than one occasion