Rashi on Numbers 8:16/רש"י על במדבר ח׳:ט״ז

1נתנים נתנים FOR THEY ARE GIVEN, GIVEN ARE THEY [UNTO ME] — i.e., given twice: dedicated for bearing the holy objects and dedicated to sing in chorus in the Sanctuary.

2פטרת means OPENING OF (such that open the womb, i. e. firstborn).