I suspect something happened in the update to OS High Sierra or one of the security release packs since. Multiple people on here are reporting assorted issues with Acrobat suddenly not respecting duplex settings - mine started defaulting to duplex, even though the "two-sided" option is unchecked in my system dialogue. I don't know how many Adobe employees handle inquiries in this forum but so far no one has offered me any troubleshooting options.
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These fora are mainly user to user fora. As there are some Adobe employees monitoring the fora, there is no guarantee that they can help more then the users. Adobe employees have the STAFF logo attached to their avatar. Users either do not have a logo (standard users) or they have an ACP or MVP logo attached. More information on this can be found here: Meet the Experts .
Now for the problem: Acrobat is reading the printers parameters from the OS. Unfortunately not all printer drivers advertise their capabilities. In that case you need to pass through the printers dialogue to set parameters that are not taken in account by the Acrobat dialogue. This is very disturbing and not efficient, but there is no other way around.