Using older versions of Outlook for Mac for personal email and calendar
I had this same issue on my Mac. I reloaded the software several times. Then I figured my problem out. I was tapping twice on the Outlook icon to get it to open and it was opening multiple tabs. When I stopped that and clicked only once it has seemed to stop having this problem. Don't know if it fixes it permanently or if it will work for you but it so far has helped me.