The Duplicate Layer command adds a new layer to the image which is a nearly identical copy of the active layer. The name of the new layer is the same as the name of the original layer, but with “ copy” appended to it.

If you duplicate a background layer which does not have an alpha channel, the new layer is provided with one. In addition, if there are any “parasites” attached to the active layer, they are not duplicated. (If your understanding of the word “parasites” is limited to small, unpleasant creatures, please ignore the last sentence.)