I'm using a mac running Sonoma 14.5 and my Quicken for Mac is up to date. My printer is an Epson ET 8550.
When I try to print more than a single page of checks, they are printed double-sided. I make sure the single-sided option is selected before printing, but double-sided printing happens anyway and when I next open the printer dialog it has reset to double-sided.
I don't have this kind of printing problem with any other application, so I don't think it has to do with any default settings on my mac.