Unique ID field?

I am using a text field (text - Text (number/time/etc/custom validation)) to enter ID Card Number. This ID is unique and there can be no other entries with that ID. An error must be shown if this ID is entered again while submitting another form.

How can I achieve this? I googled and searched this forum but couldn't find a solution.

you can't do whiout create your own field becasue you need to check if value alreday existe before save

Yes. How do I create my own field and check if a value already exists?

sorry we can do it for you
check the faq at first and you can find this
www.flexicontent.org/documentation/tutor...-new-field-type.html

The link you gave is to duplicate a field that I don't think has any relation with my unique ID field situation. I don't see how we can create a unique ID field in that tutorial.

Am I missing something?

we didn't have a tutorial for your case
the link is for duplicate field to create yours, and after adding your code to check if value is already exist