A client recently called me because whenever she tried to open an Excel document, it opened twice.
After some research, I found that the issue only occurred with Password Protected Excel sheets when the Preview Pane is turned on in Windows Explorer.
The only real option to avoid confusion is to turn off Preview whenever you open a Password Protected Excel document.
Thanks for vinnyp for the answer given here: https://community.spiceworks.com/topic/post/3918932
Although that link discusses older versions of Excel, the issue my client was having was with Excel 2016.