[whpalmer4]

You'll only get duplicates if you check a repeating action off on more than one device without syncing in between. If you check something off when you do it, and sync when switching to using a different device, it doesn't happen. If in doubt, sync first and check off later :-)

Presumably you're putting these tasks in OmniFocus to help you remember to do them. If you're just using it as a place to record everything you did at the end of the day, there's not much point in using it, as it isn't helping you choose appropriate tasks to work on.

As for automatically removing duplicates, it quickly becomes an messy problem. OmniFocus handles devices not syncing for several days, so potentially the duplicate result can be repeated multiple times, edited, etc. making it difficult to reliably stuff the cat back in the bag.