Is "remove duplicates" an add-on? Check with it's support page.

Some e-mail providers have an option to sort incoming mail. Contact your e-mail provider for help.

What email service do you use?

Do you use a web based interface to access the emails or a program like Thunderbird?

Are all messages duplicated or only some specific messages?

moved to the Thunderbird questions queue

I have been getting duplicates for some time and I believe that every duplicate I receive adds unnessarily to my very limited download quota. I have installed "remove duplicates" but that doesn't really solve the problem. My internet provider says that it is not their system causing this. Perhaps, after many years of using Thunderbird, I should now consider switching to a different email programme. fed up.

Using hotmail in one of it's many names? If so your mail provider is the problem. We are all experiencing issues because they are not deleting mail, or moving it in a reasonable time frame, so it gets downloaded a second time. There are some settings on the hotmail site to tell it to actually delete messages, but I have no bothered. I get little mail on this poorly run service. It is becoming more useless with every release.

My provider is BigPond and I am using Firefox.

If you have email accounts from bigpond and have been moved to their new billing system, you not have email accounts with hotmail/Outlook.

I have never had a Hotmail account and have never implied that I did. I have been using Thunderbird with Bigpond for 2 years now and they say the duplicates are not their fault. I am not aware of any new billing system. I have my landline and internet provided by Bigpond. Unfortunately, they have a monopoly where I live.

Can you confirm that the duplicates are actually being downloaded and not previously deleted emails reappearing ?

Are you talking about a POP mail account ? Does this only occur in the 'Inbox' folder?

It is a POP 3 account. The duplicates only appear in the inbox and it doesn't seem to matter if they have been deleted or not. Often 2 or 3 duplicates will appear before anything has been deleted.

Here is a screen grab of "received mail" BEFORE OPENING. This is as received, none have been opened/deleted etc and I haven't seen any of them previously

Where you live and where I live are not all that much apart I am sure. Here in the Finders Ranges the Telstra monopoly is fairly obvious. I share a back fence with a Telstra Telephone exchange. They told me I lived to far from the exchange for ADSL 2+.

So I know the provider (poor) I know their support (they do not even know they are lying to you.)

Their new billing system is not my terminology, it is theirs. They talk about which platform your mail is on here https://www.telstra.com.au/support/category/email/set-up/imap-pop-and-smtp-information-for-manual-email-set-up Note here that outlook.com , hotmail, MSN and live are all terms that refer to the exact same service supplied by Microsoft that has had more rebirths and names than I can remember. i really h which of their account are serviced by Outlook/Hotmail and which are done another way.

What 'I do know is my Bigpond email accounts that have existed for over a decade are not affected. My accounts with outlook are.

Basically I think the Outlook.com platform is over subscribed and with the use of the same login mechanism in Windows 10 their system is struggling. But I am sure you will not get anyone from either of those commercial enterprises to admit something like that. That would be like getting activ8 to admit the same think, instead their customers have had terrible service for 12 years.

Log in using your web mail and see if there is an option to actually tell the site to delete mails when the a mail client issues such a command. In the case of outlook.com there is such a setting. Otherwise mail is not deleted it is moved to the pop folder in trash. It is lengthy delays in processing of these moves that is the root cause IMO

Thanks for your suggestions. Using webmail, my inbox was empty (I had already told it to delete after sending) but this mysterious POP folder had nearly 300 entries (I couldn't see any duplicates in it). My webmail is "Outlook" and I certainly didn't create the Pop folder but I do keep emptying it. I have now deleted it....see what happens.

it will reappear unless you change the setting that moves mail to that folder. This url might take you to that location https://blu177.mail.live.com/mail/PopEnableDisableSettings.aspx?n=1481164912

So at the end of all this you do have a hotmail account.... supplied via bigplog.