That wifi widget is one of HTC's widgets. If you don't like it where it is, you can move it. Or if, like you say, you want two copies of it in different places, you can have that too.
To move it, just press and hold it ("long press") until it twitches, then drag it anywhere else on the screen and drop it. Or if you want it on another screen, drag it off the edge of the screen towards the page you want it on. The page will turn and you can then drop it in whatever spot.
To leave the original where it is and create a second copy of it, go to whatever screen you want the second copy to be on. Longpress the empty spot where you want it. That brings up the same screen as when you hit the Personalize button. Doesn't matter which one you do. Then tap "Widget".
The particular wifi widget you're looking for can be found in two places. Firstly it can be found in Widgets > Settings (it's the very last option) like yodatom10 said. And secondly it can be found in the Widgets > Browse All HTC Widgets. The Widgets > Settings method is just a shortcut straight to the second method, meaning that that particular widget is just one of 70 or so made by HTC that all live together in the Browse All HTC Widgets row. Tapping that brings up a gallery of widgets made by HTC. You might like to use some of the others there. The first one you'll see is a bookmarks widget but it's just the first of many. Swipe to the left to browse through the gallery of widgets. Somewhere in the 40s you'll start to see widgets that look like your wifi widget - same size and style. Keep going and you'll see the exact one. Its title is "Settings - wifi". Tap on it, and it will place the widget in the spot on the homescreen where you previously long pressed. If it doesn't land where you wanted it, just long press until it twitches and then drag it to the right spot as described above.
So the point of all of that is to say that the things on your home screens are not the programs/apps themselves. They're either just shortcuts that call up the apps or they are widgets that perform all or part of an app or setting's function right there on the homescreen - like the wifi widget. Any of them can be deleted without deleting the actual programs and can be readded at any time.
A slightly confusing thing here is that when you hit the personalize button, you can choose to add a "Widget", an "App", or a "Shortcut". Realistically in this context Apps and Shortcuts are the same thing. They're both shortcuts. But the "App" option is a shortcut to apps and the "Shortcut" option is a shortcut to settings. In this case you don't want either - you want a "Widget" and can get it by the process described above.
With all of that said, if you have updated to Gingerbread already, that wifi widget is no longer necessary unless it's just convenient for you. Because now when you drag down your notification bar from the top of the screen, you'll notice it has two tabs at the bottom. The first one says "Notifications" and the other says "Quick Settings". If you tap Quick Settings, you'll see that one of your five toggle options there is wifi. So that's an easy way to toggle it on and off without wasting any homescreen real estate on a wifi widget. If you don't see the two tabs at the bottom once you drag your notification bar down from the top of the screen, you don't have the Gingerbread update yet and will want to hang on to your wifi widget a while longer.
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