For me, "saturated" implies there couldn't exist more of given quantity, like concentration of substance in a solution. So "over-saturated" (as in "over-saturated market") seems redundant.
Yes, but there's no getting around it. Saturated is used for things that can literally "hold" more than their saturation point now, for example color in color photographs.
Where color saturation could once be enhanced by means of filters with wonderful results (polarizing filters for the sky, and water, for example), today, someone has merely to keep clicking on a button in a photo-processing app to render color saturation richer and richer until (no, the photograph does not fail to accept anymore, which would be saturation) the photo can actually become grotesquely colored.
I would say that persan would need to cut back on the saturation. If asked why, I could only say, "It's over-saturated."
I've checked my dictionary and found that the majority of the definitions given for "saturated" have to do with a maximum or greatest possible amount of something. In these cases, "over-saturated"ness would seem logically impossible.
The only possible use of "over-saturated" that I could devise (going strictly by the dictionary meanings of the word) would be when "saturated" is used to describe a colour. In such a case, the meaning is "very strong or bright". I suppose then that one could use "over-saturated" to mean "too strong or bright".
In examples such as "over-saturated market", the intended meaning is likely that the market is too densely filled or populated. This meaning is easily understood, but going by the strict definition of the word "saturated", it is nonsensical.
I think there are tons of examples where the terms differ. Yea over-saturated and saturated might mean the same thing in some contexts but not all.
Take paper towels as an easy example. If the paper towel was used to pick up water it may be saturated. If there is so much water that it cannot hold any more water and is releasing the water, then it would be over saturated.