@Marawan Elbaz As per @Michael Latchuk comment, can you please provide more information as to the business use case? If you can add screen shots to support your information that can help us Community members understand better and potentially provide you with a viable solution.
My initial thought one solution could be validating the value entered using a Unity Script and in the unity script it does a lookup to the keyword data set table and checks if the value exists. If it exists, then you can populate a hidden text box with a string value "The value entered <value> already exists" and then you can control how the text box is displayed using Custom Action etc. And then you can also invalidate the field as well using Custom Action.
As example, a customer I built a solution for them wanted a Unity Form to be able to validate that an email address entered by a user is a valid formatted email address. I used a unity script that would validate it using a regular expression.
Respectfully,
George
