Two ways to look at "oversharpening": sharpening when the knife doesn't need it and sharpening it beyond the capability of the steel.
I wouldn't sharpen a home knife all that much, occasional use isn't going to dull it unless it's knocking around in a drawer with the edge unprotected (which I am sure you are not doing!). For steel of the order of VG-10 or Hitachi Blue, touch-ups are really only necessary when the edge loses it's "magical" sharpness, which it not going to be every use.
Sharpening these knives all the time without allowing them to dull is the first sort of over-sharpening in my book.
Lesser stainless knives with soft steel should not be sharpened up through very high grit stones because they have fairly large chromium carbides embedded in a fairly soft matrix, and high grit stones will pull the carbides out of the edge. These carbides give softer stainless great abrasion resistance and allow you to reset the edge with a fairly soft knife steel since base steel isn't all that hard, it bends rather than breaks in use. If you yank all the carbide near the edge out, you are left with soft steel that will take an amazingly sharp edge for one cut, then become quite dull. Think Henkles, Wustoff, and Victorinox "traditional" chef's knives.
Sharpening these knives up to 10,000 grit and stropping on diamond is the other sort of over-sharpening, going beyond the capability of the steel.
While wearing out a knife can in fact be done, it's fairly difficult in the home kitchen unless you are in the habit of excessive sharpening events or serious over-use of a steel on softer knives. The amount of steel you remove is small at each sharpening as a general rule, and if you only sharpen when the knives actually need it, it will take a long, long time to do much damage. I sharpen even my mediocre Chicago Cutlery knives every six months or so, and that may be excessive once I get them in better shape -- the edges were pretty flat when I got them. I've sharpened my Tojiro DP exactly once in nine months, and probably won't sharpen again for quite a while, it's still cutting very nicely.
Professional use is quite a different animal, and even top quality knives may need sharpening weekly (or more often depending on what the chef wants and the knife), so it's possible over a few years to remove enough metal to alter the profile enough to change the knife. Eventually it will become difficult or impossible to maintain the basic geometry as the edge moves up into the body of the knife, and after a certain point it makes more sense to replace rather than re-grind the knife, and it may in fact be impossible to restore the original profile. This will happen faster with softer carbon steel than with high hardness alloys, but all knives will eventually become useless from sharpening.
My brother inherited his father in laws packing house knives. Many of them have been sharpened so much the profile is altered, and they all need some thinnning to work nicely. Old style carbon steel -- as the original owner said, you have to have knives that stay sharp a long time and are easy to sharpen, or you can't keep up with the work. He could de-bone a raw ham in 30 sec or less, I think, since he did it for a couple decades all day long. Wore out quite a few knives in that time, too.
One other note: use of those horrible "pull through" sharpeners will greatly reduce the life of a knife, so will powered sharpening tools. Along with removing way too much metal way too fast, they make the knife thick behind the edge quickly and offer no means of correcting this, so even if sharp (and they won't be, really) they will wedge terribly.
Peter
