Speaking from a 4000 perspective. It won't automatically duplex. Double sided should not be a problem as long as you have the right paper - You'll have to print one side and reload for the other.
I'm using dual-sided DuoBrite Matte in my 4800. I'm experienceing frequent mis-loads when the paper is fed with the curl facing down. When the curl is up, it seems to feed fine, although stacks of more than 10 sheets cause problems feeding too. I've not attempted to clean my feed rollers because it's darn near impossible to get at the rear of the printer in my installation. (soon to be changed)