Seeing Double in Photoshop
Apart from the space taken up by Photoshop's tools and palettes, the usual tendency is to fill the rest of your screen with the biggest view you can of the image you are working on. That makes sense since you want to see what you are doing. But sometimes you are better off having two copies of the same image on screen rather than just one big one in order to avoid problems. Sometimes "seeing double" can be a good thing.
There are times when making adjustments to an image it can be helpful to compare what your image looks like after making a change to what it looked like beforehand. Without this frame of reference it can be easy to get carried away with tweaking an image to the point that it bears only a passing similarity to the original. You boost the saturation or what have you and you like what you see so you boost it a bit more and then a bit more for good measure. I'm betting at least some of you can relate to this problem. I'm also betting that even more of you have suffered from it without even realizing.
To give yourself a frame of reference for what you are doing, use Image >> Duplicate to create a copy of the image you are working on. Position the copy on one side of your screen and the original on the other. Or if you have more than one monitor put one image on each. Now continue editing the original just as you normally would but keep an eye on the copy while you do so you can track the overall difference you have made. You can size the two windows any way you want, but I generally fill most of the available space with the original that I'm working on and resize the copy smaller since it's really just there for reference. When finished, you can delete the copy, or save it if you want to use it again later. You might also consider flattening the copy and adding it as a layer in your original file so you can easily make use of it whenever you want in the future.
When you are working on a small section of an image and have the view zoomed in to see clearly what you are doing you can have a somewhat similar problem. Even when your adjustments come out the way you wanted they may not fit in with the image as a whole. You are bound to encounter this problem from time to time when your monitor is filled with a close-up view of the area you are working on and you can't even see the rest of the image. For this kind of problem Image >> Duplicate won't help much because the appearance of the copy doesn't change. What helps with the first problem I described is precisely what makes this technique not work here.
To make sure that detailed adjustments in part of an image still fit in with the whole requires a different kind of second window. What is needed here is a window that shows a second view of the same image, not a copy of it. Adobe thoughtfully provides the answer by means of Window >> Arrange >> New Window for .... This will give you a second window containing a second view of the exact same image you started, not a copy of it. Whereas Image >> Duplicate creates a second image disconnected from the first, the image in the window created by Window >> Arrange >> New Window is the first image. Any change you make to the image in one window will show in the second since they are merely different views of the same image. Once you open a new window, you can move it around and resize it independently of the first. As such, you can zoom one out to show the whole image while magnifying just the part you are working on in the other. This means you can keep the overall image in view while working on your detailed edits. Yes, zooming in and zooming back out whenever you need to check how things are going will also work, but that gets tiresome quickly. Trust me, I've tried both solutions and New Window wins hands down.
Neither of these ways of "seeing double" is necessary all the time but both can come in handy when the need arises. Give them a try if you haven't already.