In order to prevent that mails are filed multiple times, e.g. several PIRS user have received the same mail from an external party and wish to file it in PIRS, there is a mechanism that checks whether a mail with the same content, subject and recipients has already been uploaded.
If you are filing one or multiple mails and one or several mails are already available in PIRS, you will receive a notice during the filing process informing you about the potential duplicates.
You are then able to decide whether you want to go ahead and file all mails regardless of potential duplicates or file only those mails that have not been filed yet.