Editable objects shown as on every layer - how did that happen? There are a number of objects in a file I've been working on for a long time that have this setup I don't understand. They show as being on all layers but they are one object. They can be edited, so they aren't objects linked from another file. 



I must have done this months ago but I can't recall how or why, and I'd like to change it. What is this setup and how do I get rid of it?
Any object can be on any amount of layers at the same time by pressing Shift when selecting the layers in the popup that appears after pressing M in Object Mode.

Just press M again to move the object to the desired layer.