OTRS currently provides no notification that we have received emails from the same address in the past. Because many people send us multiple emails, or send a series of emails with separate subject lines over a course of weeks, months, or years, agents end up sending out duplicative responses, or responding to people who should not be receiving further responses.
It is possible to search to see all emails from an address, but it's not practical to do so for every ticket. The interface should be flagging this for us, and ideally also including a list of the past tickets on the sidebar (much like it does for linked tickets).