mailchimp inserting duplicate emails

My understanding is that mailchimp is not creating duplicate email addresses in the same list – and if the email is duplicate it sends a message that the given email address is already subscribed. I’m using boxzilla to create the sign up box.
case 1 – When I use the same computer to signup an email address that I have already inserted in mailchipm list, it hangs and eventually times out – time out message comes from nginx.
case 2 – When I use another computer to signup the same email address, it actually gets inserted in the list and I end up with a duplicate email address.

Now my question – problem – is: case 1: why mailchimp is not returning any messages that email already exists and it hangs. case 2: why mailchimp is inserting duplicate email addresses.