I have noticed that I have selected in the In-app Limit to once ever, however, I am seeing users who have more than 1 View of the message.
In-app message limits are sent as part of the message content and are stored on the client side. Therefore, if a user re-installs the app or clears the app cache, they may see the message again on next launch, as long as they match the target.
To enforce that a message is not sent from the server in that case, just amend the target by limiting this in the target as well. You can use "Last occurrence of <message> View is prior to 1 day ago OR Total Occurrences of <message> View are 0".