As of now inside Adobe own technical article 'Print PDFs' <https://helpx.adobe.com/reader/using/print-pdfs.html> this paragraph clearly explains that:
"The printer driver controls the options, not Adobe Acrobat or Adobe Reader. Check your printer documentation to see what features your printer supports."
And also additional "Note: There could be a scenario when a printer actually supports duplex printing and this option does not appear in the Print dialog box. In such cases, open the printer properties to directly access this option from printer preferences."
So chances are that lost option for duplex printing might just have depended from last update in same printer driver, or rather that user might have even been unable to notice/verify that default printer was no more same one used in the past. Theoretically if considered printer was a network one that was changed recently, he would have simply received automatically new driver on his client and maybe a logon script might have even set such new printer as default without his proper awareness.