I've just started seeing an issue where I may have 2-3 files open in my single instance / multiple windows as expected, all sharing my add-ins and personal macro workbook. Then I'll double click a file from Outlook or File Explorer, and it opens a completely new instance of Excel, and reopens my add-ins and personal macro workbook, which are then read-only and can't save changes. Kind of a pain, and it just started. Any ideas?

Excel 2016 in Office 365 environment, installed on Windows 10 work PC.

Thanks!

Dan