Query editor loads same files multiple times

I think I have found out WHY the data loads multiple times.

It loads exactly 7 times and only after I merged it with a date table.

Could someone help me how I can prevent this from happening?

So to be exact:

I have a dataset with week data in format yyyy/mm in table one, and I want to add a column with the Mondays of these weeks into the table. (to be able to create a relationship to another file)

What I have done now is create a second table that holds the data yyyy/mm with a column with the Monday of that week in format "dd/mm/yyyy"

When I merge the tables the data in table 1 is loaded 7 times and I think that has to do with the date.

Could someone help me in how I prevent this, please?

Thank you!