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It is typically because you have more than one of the same monitor with identical EDID information.

Documented and explained here: https://spin.atomicobject.com/2018/08/24/macbook-pro-externa...

Possible fix: https://gist.github.com/ejdyksen/8302862?permalink_comment_i...

I used to have two otherwise identical displays, but it still does it for me now that all of my displays are different. Notably, I think this stuff happens more when you have 3+ monitors. I don't remember having as many issues with just two displays.

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