I cant reproduce your senario and after searching many documents,I failed to solve your issue,so pls create a support ticket via below link and our relevant personnel will help to handle it. https://powerbi.microsoft.com/en-us/support/
Best Regards, Kelly
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Hey @Samurize24 - I never really got to the bottom of it at that time and I was then consumed by another issue...WHICH may have solved the original one.
Have a go at playing around with the enhanced compute engine settings on your dataflow. What is working for me is having my ingestion dataflows set to optimised. On my transformation ones i have them set to disabled. The "key already exists" sounds a lot like a sql type issue doesn't it....and enhanced compute engine has a sql cache behind the scenes. (I'm still a bit confused on these matters, i've seen the screenshare from microsoft that says one thing (turn them on in transformation) and the matt roche blog seems to suggest not required for transformation - so am going with what i found actually works in practice).