Outlook 2013 with an Outlook.com account and an IMAP account yields multiple sent items

Question

There is a problem with Outlook 2013 that came up recently. I have two e-mail accounts: on outlook.com and one in my own serwer (used via IMAP). I'm trying to setup Outlook 2013 in order to use both accounts.

However, I realized that there is a problem with sent items. I'd like them to be saved on a server in order to have an access to them. Right now it works like this:

1. outlook.com account after sending is saving sent mail twice (there is a duplicate one).
2. IMAP server saves one copy (which is ok).

On the outlook.com web interface I can disable saving sent items so that Outlook (desktop app) saves the sent item only once (which is ok). The IMAP account is untouched and it also saves one copy. In that case Outlook 2013 works properly. However, every time I use web interface (outlook.com) e-mails I send are not being saved (which is wrong).

What should I set in order to have only one copy of sent item in every way I use both accounts (on matter whether I use Outlook 2013 or web interface).