I just took the same course.
At 24:57 in the video, I have separated the selected FACE to make a bumper.
The FACE is not duplicated.
When I made the Latch, I first duplicated it and then separated it, so I think you are confusing that operation.
If the FACE was duplicated and then separated, as others have answered, it would be a good idea to delete the original FACE that was separated.
I hope this helps you.
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I apologize if the sentence does not make sense because I am using translation software.
