The Best Way to Fix Over-Sharpening
Sometimes it's easy to go too far when working on an image. You change things a little and like what you see so you change a bit more, eventually overdoing things. And nothing is easier to overdo than sharpening given that you may not realize the effect of your changes until it's too late.
If you're working on only a small part of an image of course, you'll be well advised to zoom in to just that area, but when adjusting the overall appearance of an image it makes equal sense to fill the screen with the entire image. But unless you have a very low resolution camera or a ridiculously large monitor, you won't be viewing the image at anywhere near full size.
If you adjust the overall color or contrast in an image, you can basically tell what you have when viewing the image at any scale. Given this, looking at the entire image generally works fine for this type of image optimization task. If you're working on the entire image, you probably want to see the whole thing.
It's tempting to approach sharpening an image for printing in the same way. You fill the screen with the image so you can keep an eye on the whole thing while you employ your favorite sharpening technique. But if you do so, the fact that you're seeing the image zoomed out far enough to fit on the monitor works against your efforts to get the best results you can. The general recommendation is to view the image at around half scale to look at what you are doing close enough to see the effect clearly. Zoom all the way in to an actual pixel 1:1 view and you risk going too far the other way if don't know what things should look like. At that scale, your results may look good when they won't even be evident at printed resolution.
But the bottom line is that it's hard to tell the effects that sharpening will have once an image is actually printed. Practice with sharpening and printing a lot of image helps of course, particularly if all the images are printed about the same size on the same printer and paper stock. But it's a common sin for beginners to over-sharpen their images without noticing it until it's too late.
And then what are they to do? How do you fix an over-sharpened image? Can it be "un-sharpened" by blurring it a bit?
Regrettably, the answer is that no, blurring an image won't lessen the appearance of over-sharpening. It will only serve to create an image now with two problems: halos from over-sharpening as well as blurred details. The first problem doesn't go away as you add the second. It just doesn't work that way.
There really is no practical way to fix over-sharpening once it's become part of an image. Indeed, there are just two useful strategies to consider with regard to the whole problem of over-sharpening: avoid it in the first place, or do your sharpening in a non-destructive way that lets you go back in time to correct your mistakes because the changes were never actually part of your image in the first place.
If you're sharpening in Lightroom, everything you do is non-destructive. Your underlying image pixels are never altered and you can tweak any changes you want as often as you want without risking anything short of the time it takes you to do so. Everything, including sharpening, is added on the fly to your screen preview, and to your printed output or image export. As such, you can effectively change your mind on anything including sharpening without loss of image quality. The end result will be the same as if you had made the final edits in the first place and never gone through all the intervening edit steps and missteps.
Non-destructive edits are possible in Photoshop too, but the built in sharpening tools don't lend themselves to being done this way. When employed in their normal fashion, they always change the underlying image pixels. As with all things regarding Photoshop though, there are many ways to go about solving this problem.
The most common method is to create a duplicate composite layer by using "Copy Merged" followed by "Paste" from the Edit menu with the top layer selected. You can then freely sharpen this copy layer and simply drag it to the trash if you find you don't like the results. This method though is actually a rather poor one since it unnecessarily inflates your file size due to the extra layer and forces you to redo your sharpening from scratch even to make modest changes. Even though the sharpening isn't baked into your underlying image pixels, it is baked into the copy layer.
If you have a reasonably current version of Photoshop, you can select all of your image layers and convert the whole thing to a Smart Object. Then if you apply Smart Sharpening or most any other sharpening method on the Smart Object it will be applied as a Smart Filter. Doing so isolates the sharpening affects in a completely non-destructive way exactly same as Photoshop adjustment layers permit for other edits, and exactly the same as how Lightroom handled all your edits including sharpening. If you find you need to tweak your sharpening, you can double click on the Smart Sharpen layer in the Layers palette to reopen it and adjust the sliders as need be.
Ingenious methods such as "High Pass" sharpening are also possible to isolate the effects of your sharpening from the underlying image. I've used and written about them here before, but with current versions of Photoshop I generally don't recommend resorting to such methods simply because they require a series of somewhat non-intuitive steps and aren't necessary for most images since built-in techniques like Smart Sharpening on a Smart Object typically provides at least as good of results.
Avoiding the problem of over-sharpening comes from experience, and even then can happen on occasion. If you're not doing so already, learn to sharpen non-destructively. That way, even "problem images" can be kept under control.