If you talking about duplicate entries on display, you can apply "distinct" sql command.
Alternate, you can also use $e["on_data_display"] events to block duplicates at application level. (refer custom-events.php)

For insert/update checks, you can have your server-side validation before posting. (refer server-validation.php)

Hi Abu,

As you mention,

For insert/update checks, you can have your server-side validation before posting. (refer server-validation.php

server-validation.php example explain and works very nice that how it is possible to check duplicate records during inserting or editing records.

Thank you very much.
Regards….