If you are using the duplexing function in your printer driver, stop using it, because it seems to have been set up wrong.
OSX lets you print alternately the even and odd pages of your document and in forwards or backward order.
All you need to do is feed a few sheets into your printer's paper tray, marking the top leading edge of the sheets and observe which side, and in which direction the printer prints your pages. ie usually on the opposite side to the mark and with the mark up on the top edge.
The process of getting duplexed and collated printout in the right order is then simply one of printing either the odd or even test pages first, then taking the printed copies and feeding them back into the printer paper tray facing the opposite way, in most cases facing up, so that the printer then prints to the other side.
Make notes of the results. Doing a set of 6 should sort how it works, then follow your notes each time to get the result you want, which would be double sided printing, collated from front to back on the second pass.
It is not difficult, simply a physical behavior of your printer.
Peter