Try it a third time.
For I would not have known covetousness unless the law had said, "YOU SHALL NOT COVET."
So then your answer is that it is NOT acceptable for a Christian to covet if I'm reading you right.
Christians use the law as a check. We use the written law as a guide to tell us if we're sinning...or better worded, to tell us when we're not letting Christ live through us and instead are letting our own nature live.
And it happens everyday in the lives of Christians whether they admit it or not... that is, COVET.