With right click on the marked sequence and choosing duplicate in the panel, the duplicated sequence is not independently from the other. A change in the new sequence causes the same change in the original sequence. The same happens by copying everything with cmd+c in one sequence and pasting it with cmd+v in another sequence with a different name. By the way, I am using the version 2020 of premiere pro at macOS Big Sur.
Has anybody an idea, how to duplicate sequences independently?