The Scenario; You're a multitasker. You have two or more documents open in Microsoft Word (by the way, this also applies to Excel and PowerPoint), plus numerous other documents and programs running.
Multitaskers know about the Alt-Tab keystroke to cycle through everything that is open instead of using the mouse to click on icons in the task bar below. Alt-Tab takes much less time and effort.
The Problem: Let's say we have Word Document A and Word Document B open, and we are currently in Word Document A. We know in our lizard brains that only two Word documents are open, but when we Alt-Tab to travel to the Word Document B our screen shows TWO OF EACH! Often we land right back in Word Document A from which we were just trying to leave. What is happening in MSOffice's brain is that it thinks two separate documents are two separate applications. I swear I could kill the person who thought this would be useful because this has been annoying me for years. I didn't even know if this problem had a name it was such an enigma. I've attempted to Google multiple phrases describing this ridiculous state of affairs. The phrase we need is "App Switching".
The Solution: File / Options / Advanced. Then in the Display category, find the "Show all windows in the Taskbar". Uncheck this, and alt-tab will stop trying to interpret each spreadsheet in the same instance of Excel as a different application.