One email address has two Internet Accounts
You don't have two email accounts.  You just have two email protocols set up.

From Wikipedia:


  Virtually all modern e-mail clients and servers support IMAP. IMAP and
  the earlier POP3 (Post Office Protocol) are the two most prevalent
  standard protocols for email retrieval, with many webmail service
  providers such as Gmail, Outlook.com and Yahoo! Mail also providing
  support for either IMAP or POP3.


The major differences between POP and IMAP is that POP supports the "download and delete" methodology and IMAP "synchronizes" its content between the server and client thus allowing you to have multiple email clients updated.

With the setup you have, the POP protocol negates any advantage you would have gained with IMAP since it downloads and deletes the message meaning there is no longer anything for IMAP to sync.

So, which one to use?

If you want sync'd emails across devices, keep IMAP.  If you have only one device and you want to save your emails locally, use POP.

You can remove your POP email protocol by deleting it, but if you want to save your messages, you can create a "Saved Message Folder" under the IMAP account and move everything from POP there.  Once copied over, delete the POP account.