Question After subscription to categories members receives messages in 3/4 fold

It happens with all new posts.
On the sub "Users" I added all users to subscription on new posts in categories.
Then they receive 3x a mail on a new post.
When a user makes a subscription in a category in the forum only he receives one time a mail.

My question : is it not possible to include all users in a subscription via the sub "users"?

Users can remove the forced subscription in their profile. If they are nevertheless interested, they can subscribe to the category themselves.

Thanks a lot for your reactions.
But to be honest it is not an answer to my question. That users can describe themselves is clear.
My question was : is it not possible to include all users in a subscription via the sub "users"?

Whether a user subscribes a category themselves or you do it through the backend, the result is always the same. A row is added to the #__kunena_user_categories table with the category ID, the user ID and the value 1 for subscribed.

Are you perhaps using the extension auto subscription for Kunena?

I see indeed : "#__kunena_user_categories table with the category ID, the user ID and the value 1 for subscribed."
Nevertheless "Whether a user subscribes a category themselves or you do it through the backend, the result is always the same." is not for me correct.
When a user subscribes to a category he receives only one email per new post. When I add the users to subscription in the backend in the tab "users" with the button 'add to subscription' they receive 3x an email per new post. That is my question: what is going wrong?

I have no explanation how this can happen, unless you are using an extension for Auto Subscriptions. But you didn't answer this question.