Some students routinely copy off others and probably have been doing so their whole lives. The best way to prevent this is to provide each student with a different set of problems, not just the same problems in a different order, but problems with different answers.
The following are PDF files, each containing 25 sets of problems, with the answers on the last page(s).
Shopping Fill out order form. (Reading comprehension)
Prime Factors Factorize product of three prime numbers.
Units of Time Convert decimal hours to days, hours, minutes and seconds.
Long Division Fill in missing steps.
Travel Time given distance and speed. (Reading comprehension.)
New batches or custom worksheets may be purchased by
$1/batch for fresh batches (specify number of versions/batch; I can do 100 as easily as 1).
Make me an offer for a custom worksheet or test.