I got lucky and found two scanners very close to each other on a planet. If I had chosen Monolith both times would they both have pointed me to the same one? Or do they always point you to a unique location if one exists.
AFAIK, choosing the same thing on the same scanner twice will result in two different locations, so doing each scanner should return different results. I have done the two scanner trick multiple times, same scanner only with Colonial Outpost though. Each scanner also has its own unique identifier, which supports the idea that you'll get different results from each.
It has worked out randomly for me. I have never experienced the same scanner giving me the same destination when I query it multiple times for the same thing (possibly because I haven't tried it enough). However, I have sometimes experienced different scanners (whether close together or far apart) giving me the same destination. And as far as I can tell there doesn't seem to be a way to know (in advance) that you've already been to that place unless you have really good spatial awareness.