The pattern for me at the moment is that mapping planets requiring 2 probes (or low mass 4-probe planets) 'sets' the bug - after that any subsequent planet in the system will be marked as 'Mapped' (and you can check this in the left hand nav panel). However, if you then go on to scan a high-mass six, or a seven probe planet, it re-sets the bug. Jumping to the next system also re-sets the bug for the new system, but if you go back it's still the same. Logging out altogether or to Main Menu does not reset the bug.I spent several weeks trying to establish a pattern for the “already scanned” planet bug. In the end, nothing seemed to form a reliable set of circumstances, every time I thought I could see a pattern, it got broken a few scans later. Eg. I thought for a while it was connected to scanning two HMCs in a row, and this held up for a few systems, then it didn’t.. Then it seemed to be connected to scanning Water Worlds as the first scan in the system, then I thought it might be scanning an object in an “A” spectra system, etc etc etc...
In the end, I gave up and viewed each occurrence as being an opportunity to practice my memory skills for where probes landed
Also, if you scan a low mass planet with a moon in close proximity (or vice versa) the image of the probe landing on the target planet is replicated on the companion planet. Amusing as it may be, it's still a bug, and gives us a clue as to how it affects other planets in the system. In other words, it would appear that under certain circumstances the game to set the status of all the planets in the system to 'Mapped' for graphical display purposes, but in the commanders personal inventory the planet is still 'UnMapped'.
Btw I can't find the reference to this in the issues tracker - if anyone can point me to the right thread, it be grateful - thanks ! For now, I'm developing my memory skills as well