Well, it would pretty much be the same message as it is now but with another tag. Since the "Denied" message for already punished players is "The player has already been punished blablabla".I agree with this, since "Pending" is also used in the /report command. So it would be nice if those would have the same messages/words.
I don't like the "Partially-Accepted" tag, but it could just be "Handled" in another colour like: "Handled". The message should be something like: "The rule-breaker has been punished, but the rule-breaker was already reported or dealt with by Sentinel." instead of "Your report was successful."
I found it also frustrating when my report gets closed because someone was just faster or the cheater was already banned by Sentinel. You also did put the effort in creating the report, so why shouldn't you be rewarded with a successful report. But in order to get this, your evidence should also be valid.
So it would not be more effort for staff members.