I am brand new to Krita. I opened an existing JPEG picture with Krita 4.17 and am unable to copy a door and paste it to make a second door in the picture. If I use the polygonal Selection Tool to encircle the area I want to duplicate, then hit CTL+V, it creates a layer. Then what?
If you have made a selection, pressed Ctrl+C and Ctrl+V and a new layer appeared, the selected area is duplicated in the new layer. It's exactly the same as the original and in the same place, so the image is unchanged. Take the Move tool and move the copy to the wanted place.
Be sure you have the new layer selected in the layers panel before you use the Move tool. As well be sure that you have a non-empty layer selected when you press Ctrl+C
BTW: Getting automatically a new layer is something that users generally appreciate, because the original stays intact and one can easily test different edits without destroying the original.