I have encountered this type of issue before, but not with this particular plugin.
It seems that certain themes or plugins can sometimes cause a shortcode on a page to get processed more than once, usually in the header so you don’t actually see any additional output.
If that is what is happening, then my shortcode which handles both displaying and processing the contact form is getting called multiple times after the form is submitted, causing it to send multiple emails.
Either that, or people are hitting “refresh” several times after submitting the form.
Are you able to do a process of elimination to determine if another plugin, or the theme itself, is causing this conflict with my plugin?
Also, for my own information, are you putting the contact form on its own page and using the separate contact form shortcode? Or, are you having the contact form dynamically generated on the same page as the directory?
So far, you are the only one to report this problem, and I am unable to reproduce it on my own sites. If you figure out which other plugin, or theme, is causing the conflict, please let me know so I can test it on my system and try to find a solution.
With my past experience with a similar issue on another plugin, I do know a potential work around to avoid this situation if it is this type of theme/plugin conflict, but it would require re-writing a big portion of my plugin. At some point I will probably rewrite that part of the code anyway, since it is a better way of processing the form, but can’t guarantee when my schedule will allow that.