Filter posts belonging to multiple categories

I would like to filter the posts by category, showing only those belonging to both catogories.

For example, if I write: category = “1,2” and set configuration as AND, I see posts of category 1 and posts of category 2. I only want to see those posts belonging to all catogories checked. I don’t want to see the sum of results of all categories, I want to see the results that has all categories marked.

Thanks in advance.

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