Roundcube Server Not Deleting Emails After Outlook Downloads

My email account is connected to MS/Outlook 2010 in Office Professional through the POP/SMTP protocol. Within the last month, email from the Roundcube server is  being downloaded to Outlook without any filters that say "This email was downloaded already; don't do it again." I now have 742 emails in my Inbox, many duplicates, and the downloads keep coming daily. It looks as if this began on October 22, 2018. I have four mailboxes that I've connected with Outlook and only one of them is having this issue. Because only one email box is affected, It feels as if this issue is on the Roundcube side, not the Outlook side. Any ideas?
Thanks!