Skip already handled incoming emails

It seems like (at least on my machine) that every mail is processed
twice. Added a check if the email is already handled and if so, skip it.

services/mailer/incoming/incoming.go

@@ -219,6 +219,11 @@ loop:
 			}
 
 			err := func() error {
+				if handledSet.Contains(msg.SeqNum) {
+					log.Debug("Skipping already handled message")
+					return nil
+				}
+
 				r := msg.GetBody(section)
 				if r == nil {
 					return fmt.Errorf("could not get body from message: %w", err)