First of all, I would like to say Thank you for creating great plugin!
I used Contact form 7 with DB extension for a while and noticed that people press submit button multiple times while waiting for server response. (I noticed this behavior because I found so many double entry on my database)
It is easily prevented by disabling submit button during ajax communication after people click submit button.
I added two line codes in your scripts.js as following.
after line11 (in beforeSubmit event):
after line25 (in success event):
Please give me your feedback. Thank you!