Renaming files with mv without typing the name two times
Something like this isn't really necessary.  I do something like this (bash) all the time when I want to change a part of a filename (renames "foo-bar-baz.txt" to "foo-bar-quux.txt"):    mv foo-bar-{baz,quux}.txt

You can have an 'empty' bit to add or remove something from the name (renames "foo-bar.txt" to "foo-bar-baz.txt"):    mv foo-bar{,-baz}.txt

That will work with pathname parts as well (as in the linked demo video) if you include them in the command.I guess the linked script is useful if you need to do some complex edits to the filename, since you can't usefully have more than one curly-brace-group for this use case.  But in that case honestly I'm fine just double-clicking the first argument to select, and then middle-clicking to paste, and then using the arrow keys to edit.