Outlook icon in taskbar can start two outlook.exe processes

It's indeed a timing issue where specific COM Add-ins could also cause additional outlook.exe processes from being started if the first process hasn't properly registered yet.

A quick fix which works in many cases is by modifying your Outlook shortcut and adding the /recycle command line switch to it.

The target would then become;

"C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE" /recycle

For additional info see; Command line switches for Outlook 365, 2024, 2021, 2019, 2016, and previous - Robert 365

If the issue is caused by an add-in, you could verify which ones you have installed by going to;
File-> Options-> Add-Ins

See if the issue still persists when they are all disabled. If so, you can enable them one-by-one to find the culprit.