Duplicated Inbox in my e-mail account

[Pluschap]

K-9 mail insists on creating duplicated Inbox folders for one of my e-mail accounts.  It then downloads messages to both inboxes, meaning that unless I open each folder in turn and clean them both out, one of them will continue to accumulate and consume unnecessary memory on my phone.

Does anyone have any idea how to stop this from happening, or even how to delete the superfluous folder?