The Wikipedia app for Android (and iOS?) should have an option to move articles between reading lists. To get to this option, a user could go to the "My lists" tab, choose a list, and tap and hold an article on that list. The menu that appears should have a "Move to another list" option.
A similar option is already present - "Add to another reading list". The issue with this option is that it does not remove the article from the original list after adding it to another one, which can be annoying when a user wants to move multiple articles (e.g. by using the "Select" option in the same menu).
I can't speak for everyone, but I'd imagine more people would be/are moving articles between lists than adding the same articles to multiple lists.