Categories trouble

Let’s say I create the following categories:

Category 1
– Category 2
– Category 3

Okey so Category 2&3 both have Category 1 as parent.

Now if I make a post and I want it under Category 2 I just check the box for that.
Now here’s the trouble, since I choose a child category to the post, the post will now be posted double times. The first post have Category 1 and the second post have Category 2, even if I only checked Category 2 as the category for that post.

What’s wrong?

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