When producing a single file, ability to manually sort file ordering

[sandrock]

As-is, the --single-file option will create a single HTML file with all source files in arbitrary order.

I would like some files to be in first position (README, Index, and the rest).
One option here is to specify those files first and the the directory:
mdbook --single-file mybook.html help/README.md help/Index.md help/

Or I would like a fully-specified ordering of files.
mdbook --single-file mybook.html help/README.md help/Index.md help/file3.md help/file4.md
In this case I can do a bash script to keep this list in source control.

Does mdbook use the file spec in order?
Does mdbook avoid outputing the same file many times when specified by file path and by directory path?