Good to see you, Julianne!
i'm a developer in the same team with Sophie. Our project is built by Vue. And we track the events using GTM. So, the initialization is called twice with same library name before. So, as you said, we remove all the codes about Mixpanel in Vue. But, same cases are happening in our service. (First event has a different device id.).
I know that the initialization is called automatically when i track the event.  So, the only thing what i did in this project is creating the tag using GTM Mixpanel Template and triggering it.
Or, does reset() method make this problem? The project must call reset() method when a user enters the project.
I change the code now to call reset() when users logout only and monitor the problem.
Below is the project we had problems...
https://mixpanel.com/project/3846686/view/4342429/app/events
Thank you for your cooperation!!! 🙂