Hello Billannem, welcome to the Seven Forums.
I guess you are using Outlook, for this is a not so rare "feature" of it.
The most common reason is when original messages are left to the server. You said this is not true in your case, so let's move to the next.
When Outlook gets mail from the server, it closes the connection after all new items are read, and only then deletes the originals if user has chosen this option. Sometimes it happens, that an external application, firewall etc. causes a short break in connection before it's properly closed. When this happens, Outlook continues after the break to read the messages until the last one, then reads once more those already read before the break.
Conflicting rules are one common explanation. For instance, you have a rule where all items containing the word "Mom" in the sender name or address are moved to a folder "Family", and you have another rule where all items containing the word "Dad" in the message body are moved to that same "Family" folder. In this case, if your mother sends you an email, telling something about your dad, the message is copied (moved) two times to folder "Family" because it fulfills both rules.
Sometimes you have two email aliases pointing to same POP account. An example from my life: my mobile phone operator has a system, where every mobile phone number automatically gets an email address email@example.com
, (xxx-yyyy-zzzz = phone number). The user can then create an alias for that same email account. My Finnish name Kari is not very common here in Germany, so firstname.lastname@example.org
was free and I took it. Now these two email addresses point to the same POP account; If I set up both accounts in Outlook (pointless, but people often make this mistake), when Outlook gets new mail, it reads every message twice; once for email@example.com
and once for firstname.lastname@example.org
Today more and more people are using smartphones, synchronizing Outlook calendar and messages with phone. When synchronization is ongoing when you receive new mail, it's an almost sure method to get some duplicates.
Maybe this helps you not, however these are the most typical reasons for duplicate emails.