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.