Unique ID field?

No no. That's not what I want.

Let's say I submit a form successfully:
Name: Micheal Byrne
Gender: Male
Phone: 0000111222

NOTE: Phone field is the Unique ID

Now, if I want to submit another form like this:
Name: Kate Winslet
Gender: Female
Phone: 0000111222

An error needs to be shown and disallow: "A form with the same phone number is already submitted. Please use a new phone number."

as i say this feature isn't exist you need to code it, but don't modify existing field (this will be destrucrt by update flexicontent
create your field
adding a sql request to check if value existing on save event in plugin

oh okay. Can someone from your team help with customization, so that it gets included in the next update too and I don't lose this customisation.

What would be the charge?

same reply we didn't think its a commun request feature, we didn't add this in flexicontent core ..
you can try to contact ggppdk by mp for a custom dev

Okay. Thank you.