Side ops completed yet still available?

First off, any story-critical Side Op (those under the "Important" tab, and marked on the map with a yellow circle) will never be repeatable. These side ops trigger story events. Some standard Side Ops will also not be repeatable, such as those that are concerned with unique items (e.g. Blueprint Acquisition).

As for previously-done Side Ops (excluding target practices at Mother Base, which can be completed once per visit to Mother Base) showing up as available again, the game has a number of different "zones" within the same location, and each zone that you have story missions (completed or not) in or near should have one side op available at a time. When determining which side op in a zone should be available, the game should prioritize Important Side Ops, then any not-yet-completed side op, and if there are still no valid side ops, it will pick an already-completed side op in that zone for you to do again. The game ensures this is correct every time you are in the ACC.

Doing a side op a second time will still get you the same GMP payment (and potentially the same additional bonuses) no matter how many side ops have been done before it, unlike Story Missions (which have a 50% GMP reward penalty until you do multiple other Story Missions), so if it's an easy Side Op to do, feel free to do it and get the GMP.