identical duplicate account (username and emai)

I use AJAX Register v3.67 (the latest version at present) on my Joomla 3.4.8 website. It seems works fine until today I found a weird thing:

When I checking my email box I got two notification emails for the same person about his user account created on my website. I was confused why there are two notifications, so I went to my Joomla backend and searched his email address in "user manager", I got following result:





From above screenshot, you can see that there are two user accounts use the identical SAME username and email, ONLY the user ID is different.

Please notice the registration time, only 2 seconds interval between them. That is impossible for a human visitor to submit the registration form again in 2 seconds. So this must be a problem of one Joomla extension on my website.

Maybe this is a bug of AJAX Register? Or maybe it was just affected by some other extension, such as JCH Optimize or JotCache ?

Hope you will help me to solve this problem .

Thank you.

Yes, it's strange. AJAX Register should not create 2 identical registration and I've never seen anything like that. If you manage to replicate and think it's AJAX Register please contact me again with the steps to reproduce.