I also have a Docuprint CM305df (Running on Mac OS Sierra 10.12.5).
My problem is similar, but a little different however.
When attempting to print double sided, the following occurs.
After selecting '2-sided' in the print dialouge box, the document attempts to print.
The printer prints the latter of the pages first on the paper, the printer then retracts the paper back into the printer (I presume to print the rear face of the page), but then the paper gets jammed in the printer before it can print the other face of the paper.
Its wierd because it has no problems printing the first face, but as soon as it feeds back in automatically it all jams up.
Sometimes there is another new piece of paper trying to come out of the printer while the first page is still waiting to be printed on the remaining side.
Is this a printer issue or a setup issue?
Thanks.