While Working on Her Computer, Robin Opens and Uses One

While working on her computer, Robin opens and uses one program after another. As she uses each program, she creates and saves a document before opening the next program. She keeps each program and document window open in case she needs to return to that program and document. Occasionally, one program document becomes hidden behind another program window and she can't find the program and document she needs to use. As a result, she sometimes opens the same program and document twice in two different windows. Which Windows feature can Robin use to locate and return to a previously-opened program window, and also help prevent her opening the same program and document more than once?