This specific example isn't a bug - if the same item is completed on multiple devices between syncs, they each generate a next action. After the syncs, each device get the ones created by the others.
Actions don't currently have any memory of which ones they came from or created when they were completed - they're just independent units. It is technically possible to add that sort of relationship and use it to try and trace the lineage back to the common ancestor so we can handle this like a sync conflict. Keep in mind that doing so comes at the cost of making syncs more complex and thus slower.
As always, folks that want a feature added can email the
support ninjas and ask to be added to the item in question in our development database. Thanks!