This happens when the catch-all mailbox address at your ISP also appears as a recipient header in incoming emails. POPcon correctly identifies both the real recipient and the catch-all address, delivering to both.
Fix
In POPcon's Advanced configuration page, use the "Remove string from start of recipient addresses" feature. Enter the catch-all mailbox name (e.g. catchall). POPcon will strip this prefix before matching recipients, preventing duplicate delivery.