Sometimes you will can experience problems with your e mail client that will result in you needing to perform some maintenance to get it to work the way it should.
If your sent box becomes too full you can experience difficulties in sending mail.
You should create a new folder in your email client and move your old sent items into this if you really need to keep these.
If your sent box is too full it can result in multiple copies of emails being sent if the sent box is too full.
Also check if your mail client has an archive function ( such as outlook ) which should be run on a regular basis.
Last thing is to adjust your virus scanner to NOT scan e mails that you are sending only one's that come in.