Is there only one recoverable item per planet?
Solution 1:
Here's a Scanning Guide which shows what you can get from each solar system and planet. There does not appear to ever be a case where there's more than one collectible per planet.
Solution 2:
I believe that I collected nearly all items that you can find by scanning in my first plathrough, and there was never a case where a location contained more than one recoverable item.
Solution 3:
You could also find this out by scanning the system and monitoring your % completed statistic.
Both the scanning guide and the personal experience methods could potentially be invalidated by some future DLC and this would provide a fall back method for you.