How to stop OKI C844dnw duplex printing?

[RP53]

I am using a Mac - how do I stop the printer (OKI C844dnw) printing double sided? The only reference I can find in any print dialogue is in the 'printer' option where there is an option 'double sided' - this is off. I cannot seem to find where else this infuriating option is manifesting itself. Help please before I fetch my sledgehammer. Hopefully there is a way to completely disable this as I would never want to print double sided.