Hi @willclein ,
Please check whether there are some columns in the tables that have the same field name. If so, please rename these columns to enable each column has its unique field name.
You can also refer the following similar case that could help you:
Solved: Unexpected error: An item with the same key has al... - Microsoft Power BI Community
Besides, you can update to the latest desktop version to see if the problem still exists.
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.