There's nothing wrong with your PC. The fault almost certainly lies with your e-mail provider (or with their connection to the internet).
When someone sends you an e-mail, your e-mail provider's server sends a message back to the server which sent it, to confirm receipt. If no confirmation is received after a short period of time, the originating server sends the message again and, once again, waits for an acknowledgement. If no acknowledgement is received, the message is sent again and . . . (OK, I think you've got the idea). The number of attempts varies between different servers but eventually, if no acknowledgement is received, the sending server gives up and sends a 'bounced mail' message back to the person who sent it.
It seems that, for some reason, your e-mail provider's server is able to receive incoming mail but unable to reliably send the confirmations back. (So, the sending server thinks it hasn't arrived and sends it again). You could try contacting your e-mail provider to alert them to the problem but, usually, these problems are sorted out within a few days anyway.
Chris
(PS: The fact that you're only receiving two or three copies of each mail suggests that your e-mail provider's server is still able to return a confirmation message occasionally. I once had a friend send me mail when my e-mail provider had the same problem with acknowledging messages. Unfortunately, his e-mail provider had obviously set their server to 'unlimited tries'. Instead of giving up and sending him a (false) 'bounced mail' message, his server continued to send the mail over and over again - all 3000 copies of it!)
Chris