I haven't messed with this for about 10 years, but
off the top of my head, here's what I would try first.
Code:
Private Sub cmdSouthStoreSnacksTallySheet_Click()
DoCmd.OpenReport "wksSouthStoreSnacksTally", acViewPreview
If msgbox ("Do you want duplex?",vbyesno) = vbyes then
Application.Reports("wksSouthStoreSnacksTally").Printer.Duplex = acPRDPHorizontal
else
Application.Reports("wksSouthStoreSnacksTally").Printer.Duplex = acPRDPSimplex
end if
DoCmd.PrintOut
DoCmd.Close
DoCmd.OpenForm "frmOpening", acNormal
EndSub