SET replaces my keys and values instead of adding new ones

I have a weird issue that I can't seem to solve. I am trying to sum daily stats from GMB and aggregate them by month and category in a new object.

The issue is that SET seems to replace my keys and values instead of adding new ones even if they don't have the same name.

Here is the code :

and the result :

How can I make sure that each entry registers as a new key and value instead of deleting the previous one ?

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