If the synthetic key is happening because two or more fields are named alike in two or more tables, the most sensible way of making sure that you are loading the data as you want is to delete those fields repeated (for example, date in header and line, and you only need one). Another option if you are developing and not sure how to associate tables is renaming fields using AS in the script:
LOAD Field AS Field.1
FROM ...
Once you are sure about how you want to associate your model, then you can decide to JOIN, CONCATENATE, KEEP, etc.
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