Helping people with computers... one answer at a time.
Getting multiple copies of email is most often a problem at the receiving end. We'll look at both sending and receiving, and the most common causes.
When I send emails to a friend he says he always gets two emails of the same message. As he is on dial-up this can be very time consuming. Is the problem at my end when I send, or his end when he receives?
If it's only your email that causes this problem, it would seem to be your issue. However if you're not seeing any errors on your end when you send, it's going to be very difficult to diagnose.
However if it's not just your messages that are getting duplicated, then the issue would appear to be on the recipients end. The good news here is that there is an occasional configuration issue that can cause this behavior.
You haven't indicated which mail program you and your recipient are using. That makes a big difference. If you're using some kind of web mail interface, such as Hotmail, GMail, or even a web mail interface provided by your ISP, it's extremely unlikely that the issue is on your end.
If you're using a traditional email program, like Outlook, Eudora, Thunderbird or the like, this problem can sometimes happen if there's an issue uploading your message to your mail server. The mail program will attempt to send it by uploading the entire message, and then wait for confirmation from the server that it was received and sent ok. Sometimes the server will take too long, or that confirmation will be lost or garbled. Your mail program then assumes that the mail was not sent properly, even though it might have been, and dutifully attempts to send it again. If both attempts actually work, then two copies of the mail result.
The most common cause is a message that is excessively large.
Now, I'm skeptical that this is your issue, because you make no mention of there being any error message. Most email programs will display an error message in the situation I just described. If your email program supports any kind of logging, you might enable that and see if it sheds any light on what your program might be doing.
It's more commonly a problem at the recipients end - particularly if they're using a traditional email program on a dialup connection. If they're using a Web based email program, then it's very unlikely the problem is on their end.
When a traditional email program receives mail it requests each message individually from the mail server and upon completion of the message tells the mail server, in effect, "OK, I got this one, move on to the next". If for some reason that response is somehow lost or not received soon enough by the mail server it assumes that the message was not received properly and needs to be downloaded again.
Once again the most common cause is a message that's excessively large.
Now there is one configuration scenario that, while not common, can easily also cause two copies of messages to be downloaded. I've had it happen myself.
If the recipient has one email account, but more than one email address that delivers to that account, it's easy to configure some mail programs to mistakenly attempt download mail twice.
This can quickly get confusing because technically an email account is not the same as an email address. An email account is what's used to establish the connection to your mail server and authorize you to download mail. It also identifies which collection of email is yours. An email address is simply how people direct email into your account.
Let's say I have an email account "leo" at a domain "example.com". I'd configure my mail program appropriately, and email sent to "firstname.lastname@example.org" works pretty much as you'd expect.
Now let's say I have an additional email address "email@example.com" which I use for all my email mailing list subscriptions. The 'trick' here is that this email address just automatically forwards to "firstname.lastname@example.org". So while I'll receive all email sent to "email@example.com", I do so because it all gets forwarded to "firstname.lastname@example.org". I don't need to make any modifications to my email client. So far so good.
If I ever want to send email from "email@example.com", we run into a problem.
So far my mail client only knows about the email address "firstname.lastname@example.org", and when I send or reply to any email, that's the email address it uses. In most email programs, if I want to send from an additional email address that's delivered to the same email account, I need to configure an additional "fake" account. That fake account uses all the same account information as my primary real account, "email@example.com", but with a different email address, "firstname.lastname@example.org". When I use that fake account to send mail, the From: address will be "email@example.com".
The default, of course, is for that second account to download all email. But it's configured with the same account information as the first, so it tries to download the same email. Some mail programs allow this to happen in such a way that all the email could actually be downloaded twice. My experience was with Microsoft Outlook, for example.
The solution to this confusing situation is to change the configuration of your email program to not download email for that second, fake, account. Exactly how depends on the email program, of course. You'll still get mail sent to that address, because it's all being forwarded to your primary account. You only need that second account to send email when you need the "From:" field to be set to that second email address.
Given how confusing it is, it's no wonder that it's so easy to have errors like this crop up.