So long as you are printing on paper and not on labels or any other stationary that could be affected by the heat whilst printing, then you should have no problems. Like woodchip, I have the ml4500 and this prints duplex fine.
Make sure the paper is quite cold after its first pass before you re-insert. If you're doing more than one sheet, flick or fan then to make sure they're not sticking together.
My Samsung ML 1210 has a front exit to allow the correction ordering of paper for re-inputting to print the second side i.e. first page at the bottom of the stack. The nomal feed gives the correct order for one side i.e. first page first.
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