In Usermin I have two folders with names related to Sent:
As I see using Usermin, [Sent mail] folder keeps all sent emails composed in Usermint.
What is the purpose of [Sent] folder? Can it be deleted to avoid confusion?
Did you try setting that email account up on a client, like Microsoft Outlook or Mozila Thunderbird or the mail app on a smartphone? Sounds to me like you have two folders for it because another client created it instead of detecting the existing one.
If that is not the case, let me know. It could simply be that usermin was told to use a different folder then what Virtualmin was told to create.
However. yes you can delete one of the folders though to answer your question. However it may come back if you are using another client to access email.