Unique Email validation allows two identical entries

I am using the latest ver of DB in CS6 to setup a basic registration system.

Problem:
Despite selecting unique email validation in the setup wizard, two identical email addresses can be inserted.
However, any further attempts to register with same email are prevented. It seems to validate only after the second attempt.

This issue affects all email addresses.

Observations:
A record only appears to be inserted if it is adjacent to its duplicate in my sql table, In other words if i attempt to register a duplicate one after the other.

Where if i attempt to register an email, say two or more records after the duplicate in my sql table, the validation works and rejects the entry.

Example insert table:

id email
-- -------
1 email1@host.com
2 email1@host.com --- not rejected by validation
3 email2@host.com
4 email3@host.com
5 email4@host.com
{6 email2@host.com --- will be rejected by validation}

affected .php file attached.

Any thoughts?

Leave the key value field set to 0 in the key value field validation UI.

Thanks ray - appreciated the heads up.
That seems to have resolved it - got spot!

That field is there to allow it to work on update pages where the current record would not be considered a repeat since it is the same record.