It's possible to make duplicate keys by hand at home, if you have the right tools and some patience. This is often an exercise locksmiths do as part of their training. You should only duplicate keys that belong to you and that unlock your property. Anything else may get you in trouble.
Hold the original key over a match or flame just long enough to get some soot on the original key. When you file into the original, stop when you see the black soot.
Clamp the copy of the original key and the key blank side by side in the vice. Only use the original key if it's not a critical key. Make sure you have easy access to the blank for filing. Tighten the vice.
Know that a key has "mountains" and "valleys." Begin filing the blank to match the top of the "mountains" on the original key.
Keep the two keys lined up in the vice as you begin to file the "valleys" by hand. Use a small file to duplicate the exact shape of the original key.
Insert the key in the lock to test it. You may need to do some more gentle filing to get it to fit the lock exactly. It's important that the shape and slope of the mountains and valleys be exact in order for the key to work properly.