Duplicating frames

hello, i am trying to make an animation where the doors basically close and then open, but i don’t want to have to duplicate and drag each frame at a time. is there any way to select certain frames and then duplicate them?

sure, select frames/cels you want to duplicate, ctrl+drag them or hit alt+d or go to frame → duplicate cels. then hit alt+i or go to frame → reverse frames to reverse them.