I think you mean to ask Ai for having a duplex control right in the Print dialog, am I right? :)
Then this is not a bug, but a feature request, a good one. I am moving it to the dedicated branch and adding my own vote. 'I have always been convinced by convincing arguments'
Please keep these coming.

@Egor,
No - this isn’t about whether or not Adobe Illustrator ‘ever had’ duplex. Bugs aren’t always about what something had. A long time ago Adobe decided to take the print process away from the Operating System - and, in general, it does a reasonable job - but in doing so it must reflect upon the features (including system defaults) of attached printers. Not everyone uses Illustrator in a design department with many printers. Increasingly so, designers are WFH and need to proof their designs on a shared printer or office printer that is not dedicated to AI. So, depending upon changing printer defaults via, eg, CUPS is not a fix - it’s a very poor workaround.

Adobe probably knows more about printers than any other non-OS company there is - and not to even think to inquire about (and turn off duplex - which, as you say, isn’t really supported) is a huge bug.

Considering the huge market fall in AI usage, with Inkscape becoming a very worthy free contender, I am surprised that Adobe doesn’t wake up a bit to the change in the user landscape.

Ben, can it be the printer being the one who has the duplex option set by default? Ai never had a duplex options, so it’s hardly its fault... I had this exact problem before and it turned out someone had changed it manually on the device.
Does it help if you change the printer you are printing with? (if you have more than one)

While it is possible to override the default settings via CUPS admin, this also affects all print jobs from everywhere, which is not a suitable solution. It's a workaround, to be sure, but it's does not transfer this issue from being a bug to being a feature request. Why? Because changing a server default is not a solution for managing a given print job. Moreover, it is mysterious to most users as to why they would need to use a cups default given two separate UI panels, one of which states that two-sided printing is off, and the other - despite having six pages of print options, neglects to consider duplex/two-sided printing among them.