If I've understood you correctly than I believe your plan is unnecessarily complicated. If you simply place two copies of the same question in the same set they will be given different seeds -- so the student will see two copies of the problem that differ only in the exact numbers that appear. Is that what you want?
To do this simply create a new blank problem in the homework set (using the homework set editor), copy the "path" of the problem you want to duplicate and paste it over the path of the blank problem. Save the result. You can reorder the problems in the set to place the duplicate questions wherever you want.
When you view this set in the homework editor you will see a warning message that the same question has been used twice -- but there is nothing wrong with that. The warning is there only because most of the time two copies of the same question means the instructor made a mistake in picking problems.
There is a very, very low probability that the two random seeds chosen for successive questions are identical -- but I wouldn't worry about that.