Duplicate email #1 – a message on the server is corrupt, and thus generates duplicates when Outlook starts its Send/Receive process. In order to fix this, you or your IT administrator will have to log in to the email server and delete the corrupt/damaged email that is creating all the duplicates;
Duplicate email #2 – after you have synchronized your Outlook profile with your phone/tablet, something went wrong – the result is the last batch of emails are present twice on your PC. You can’t actually fix this, only manually delete those duplicates and hope that it won’t happen again.
Duplicate email #3 – the “Leave a copy of the messages on the server” box has been checked on your account settings and Outlook creates duplicate emails at each Send/Receive. The fix is to uncheck that option (you can find it under Account Settings -> More Settings -> Advanced).
Duplicate email #4 – you can define rules in Outlook that generate duplicates, for example:
Rule 1: move messages from contact X to folder Y
Rule 2: move messages that contain WORD in the subject line to folder Z
Now, if, you receive a message from contact X, that also contains WORD in the subject line then you will have the same email in folder Y and in Z. You have to make the appropriate changes to your rules set-up and sift through all your folders to delete those duplicate emails.
Duplicate email #5 – if you import emails from different Outlook data files (PSTs or OSTs) in your current Outlook profile, then the emails that are both in your current file and in the one you import will be displayed. Of course, if the same emails are present in both data files you will see duplicates. This is especially true for large data files, that hold years of work and the only way to go about it is to search and delete each duplicate manually (yes… a daunting task, but one that needs doing);
Duplicate email #6 – if the email server or Outlook fails to respond during a Send/Receive, then the next sync will download the previous emails again. Yes, manually deleting those duplicates is the only thing you can do.
Duplicate email #7 – if you have more than one alias defined for the same email account in your Outlook profile, you will receive the same email on each alias (you will see as many duplicates as you have aliases). To fix this, use only one alias/email account and delete those duplicate emails.
Duplicate email #8 – on an IMAP account, if you have set-up Outlook to retrieve new emails automatically upon startup and open the Inbox automatically you will see each message is duplicated. If you have only one account defined, then uncheck the option to perform a Send/Receive at startup, if you have more accounts defined uncheck the option to go directly to Inbox at startup.
Duplicate email #9 – if you forward your emails from one PC to another then the forwarding rule can create loops where messages are continually bounced back and forth between them, creating an infinite number of duplicates. You will have to change your rule accordingly (just test different configurations out).
Duplicate email #10 – duplicate emails can be also caused if you subscribe multiple times to a mailing list. In this case you will just have to contact the persons that send you the emails and inform them of your issue.
Duplicate email #11 – if you are having network or server problems, or your computer is really slow, then at one time or another you will surely come across duplicate emails. This is because most of the systems employed are conservative (if the server doesn’t know for a fact that you received the email, it will send it again… and again… just to make sure). There is actually no quick fix for this – if you are running your own email server then contact your IT administrator; else, you will have to wait for the network problems to be resolved.