Hey Dennis,
What you can do is create a checkbox in your end product that holds the value 0 or 1, 0 being that the specific user has not seen the notification and 1 being the user has.
As Start object for each user, it can be a Task. This task is a Starting point of a process flow. The task doesn't need to do anything. There should be a process procedure after this Task deciding it should continue the flow or stop based on the "have I already shown this message”-checkbox.
The Task is followed by the message you want to show. You can choose to have the message have multiple message options but if it's just a one-time throw, you can set a process procedure after the Message to set that checkbox to 1; to indicate the user has seen it.
Hope it helps!