You can sometimes get duplicate email arrive in the Inbox if its a eg: POP acct “that doesn’t completely download all the mail from the server”, where the internet connection stops randomly due to either it maybe not stable enough or the mailer gets interfered with by a background program etc, so the mail client then will “start receiving the same email again when it connects next” until it completely receives all the email from the server and clears the server mailbox.
If you are getting duplicated email with an eg: IMAP account (which is rare), that can sometimes happen if eg: the mail client “is taking too long to synchronise with the remote inbox” due to again either Internet connection delays or background programs possibly interferring etc, and the mail client can then initiate a new IMAP session, resulting in duplicate items in your Inbox.
There is also many other reasons you can also getting duplicated email arrive in your Inbox.
Now “if its only happening with one specific account and never for your other accounts” and you don’t ever see any duplicates online in that specific mailbox, then “i would suggest to just remove and re-add that account and see if that fixes it” as could be just a possible corrupted acct.
If it is a POP Mail account, first create a folder down in Local Folders at the bottom left of eM Client called eg: Old Mail, and then drag / move all your current POP account messages and any custom messages folders under Old Mail. Then go to “Menu / Accounts” and remove your old POP account. Then re-add your same POP acct via “Menu / Accounts / Add Account / Mail / Other”. Then once the account is backup and running, “drag / move all your Old Mail account messages and custom message folders up to your new POP account”. Then see if that fixes the duplicate mail Inbox problem when you next receive new mail.
Note:- If you do re-add a POP account at the end of the manual account wizard, you need to choose either to “create a Separate folder tree for your messages” or “to combine them all together” if you have eg: more than one POP account. So remember to choose as you carn’t change later unless you remove the whole account and re-add it again.
If you have an IMAP, Exchange, Office 365 or iCloud account, just remove and re-add your account also via “Menu / Accounts” either using the automatic account wizard at the top or manual option via “Add Account / Mail” depending on your providers setup configuration, and all your messages and message folders will come back when eM Client next syncs. Your messages will be initially slower to read again until fully downloaded and cached again.
Also if you do remove and re-add your account before doing anything, make a manual backup of eM Client via “Menu / Backup” just incase you need to restore for any reason. You can see when the backup is complete in Show Operations by clicking the dropdown on the right of Refresh.
Lastly also check your eM Client is up to date for whatever version you currently have, just incase you have an eg: outdated or older version. You can see and download all the latest versions for Pc and Mac via the release history page. If you do update to a later version, close eM Client before updating and also backup first incase of any problems.