There is some retry logic that by default tries up to 3 times to send an email, but it only happens when it traps an actual error so that should not be the cause, bu tmaybe your mail server is returning an error but sending the message anyway. You can disable the retries by adding this to your user.config file:
<add key="TimesToRetryOnSmtpError" value="0" />
Then touch Web.config to make it reload settings since changes to user.config are not automatically detected.
Hope that helps,