I have a Brother printer, and I suffer from the issue of printing on both sides of the paper despite having duplex printing OFF.
Today I printed from the iPad, to the same Brother printer, and the issue does not exist. It printed on only the one side.
I’m not sure if the team is aware that it works correctly on iPad, but perhaps the solution to the desktop version issue can be lurking in the iPad version.
Just thought I would share this finding.