Select the workspace you want to use, then from the menu choose Reset Workspace to return it to the way you saved it....
I did that and it returned it to the default, but the windows that I’ve added and arranged are not there. I guess that's better than having the Printing and proofing windows on both. Any idea how it may have happened?
Did you save the arrangement as a custom workspace? You would need to do that to preserve the customization.
The usual way things get messed up is we tend to open windows that aren't showing, and when we quiot ID they get added to the "current" workspace configuration, so they would appear any time you select that workspace.