LINQ: Duplicate key after GroupBy

Hello,

I have the following problem: I get an ArgumentException in the following code:

https://paste.mod.gg/jmzxdwctfzpw/0

The exception in the client system is: System.ArgumentException - An item with the same key has already been added. I don't understand how this could happen, because Im already grouping the values by the userid and this is also the key for the dictionary. So "GroupBy" creates multiple groups with the "same key"? I don't have access to the client system, so Im looking for potential causes for the exception. I am working with EF Core.

Thanks for helping.
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