outlook calendar invite that is send to one specific person gets send to multiple outlook users

Open Outlook for the user that the invite is being sent to.  Click on Tools...Options...Delegates tab.  If there are any Delegates listed, click on their Name and then click Permissions button. Make sure that the "Delegate receives copies of meeting related messages sent to me" is NOT checked.

I got an update from the user and it seems that it happends with all users. So if any user, sends a calendar invite to someone, everybody gets it?
Doesn't seem to have something to do with delegates, or does it?

Check Tools...Rules and Alerts.  Make sure the user didn't inadvertently create a rule that is forwarding his appointments to all users.  I assume it is only one particular user that this is happening with.

as in second comment:

I got an update from the user and it seems that it happends with all users. So if any user, sends a calendar invite to someone, everybody gets it? At least in one office. NOT for everyone on the same mail domain.
Doesn't seem to have something to do with delegates, or does it?