Two sided printing missing even if not using a PDF form.

you dont write what printer model number you use, but if it was me I'd head to their site and see if they had

a new driver for Mojave

apple do not write device drivers for third party hardware and if the manufacturers don't bring a new version for testing they don't include the new driver in the os install which mean one has to make do with the older driver from the previous operating system which may not work or may not work fully

Are you using AirPrint or the manufacturers printer driver? Sometimes, the setting is hidden in another location of the print dialog.

On some printers, you have to go to the settings for the printer and enable the duplexer. (Look for Options in the Printing & Scanning system prefs)

Set Up this New Printer as a New User

If everyone is using this printer, and all they get is the same settings, then it is a setting directly on the printer itself, or on each user.

So, to determine which is the official case, do this;

1. Login: as an Administrator
2. Create: a new Administrator User (System Preferences > Users & Groups)
3. Create: a new document with two pages (use one hyphen on each page, to conserve ink)
4. Print: the document
5. Check: the document output

If there are two separate pages, then it’s a setting in the user account itself. If not, check the settings on printer itself (and on the print server, if applicable)

To use any advanced features, you will have to find and install a vendor-provided driver for your printer. Then re-add your printer using the new driver.

I don't know what "many others" have a problem with. My printer had a duplex option before Mojave, and it has one after installing Mojave.

If your printer driver doesn't work with Mojave, you need to contact your printer manufacturer as they are the ones who created the driver or make their hardware compatible with AirPrint.