Duplicate entry for key. even though it is not
This is because the maximum possible stored value for the int(11) (32-bit signed integer) Contact column is 2147483647.

MySQL is silently ignoring the fact that the value 9645904409 is larger than this &amp; is "truncating" it down to 2147483647. Multiple values truncated to 2147483647 are causing the unique constraint violation.

If Contact is supposed to be a phone number, I suggest changing the datatype to VARCHAR(20) or similar.