It's not something which you can directly fix yourself. The problem is almost certainly somewhere on the 'backbone' of the internet or (more likely) with the server of your e-mail provider.
When someone sends you an e-mail, it often passes through a lengthy string of servers but, to keep things simple here, I'll just refer to the 'sending' server (i.e. the one owned by the e-mail provider of the person sending you the mail) and the 'receiving' server (i.e. the one owned by your e-mail provider).
When the sending server sends an e-mail, the receiving server sends another message back, to acknowledge that it has received the mail. Sometimes there can be a fault in the system, so that the e-mail gets through but the acknowledgement doesn't. The sending server thinks that the e-mail didn't arrive, so it sends it again. Once again, it waits for an acknowledgement. If it doesn't receive one, it sends the mail again, . . .and so on.
As I've indicated there are usually a whole string of servers involved in the transmission of an e-mail. An acknowledgement should be sent and received at every stage. If just one acknowledgement doesn't get through, you'll receive a second copy of the mail.
Sending servers are configured to try a certain number of attempts at sending the mail before they give up and send a 'bounced mail' message back to the person who sent the mail. This can vary between 2 or 3 tries, up to several hundred. (I've received hundreds of copies of the same mail on several occasions). Even though you might receive many copies of the mail, the person who sent it might still get a 'bounced mail' message because an acknowledgement was never received by the server at their end.
So, as I've indicated, there's nothing you can do about it directly. However, if the problem persists for more than a few days, it might be worth notifying your e-mail provider; it's possible that the fault