isnt that to do with the hosting you have as they set the timings for emails being sent?
also in Universal Email you can set the time delays for emails being sent, so that may help.
in Universal Email on the first part of the process click on the TO button then a new interface opens. you will see options for "Email Limits"
you could set the "wait" for 2 or 3 seconds (the default is 1 second so that could be the reason why you getting duplicates) that could solve your issue :)