Regarding validation you can achieve it by two different ways one way by configuring the validations from the properties page of the entity,
And the another way is by using microflow on the save button of that new edit page..
In this microflow you can check whether the value for that attribute is entered or not by using the decision box and then you check whether it is a unique value or not by retrieving object from the same entity by using same attribute value as xpath.
Below image is for your reference
Hope this helps you!
You can add a ‘Unique’ Validation rule in the entity:
Validation Rules in Mendix
You can simply attempt to retrieve your entity with the inputted value – if it is already in the database then it is not unique. Your XPath would look something like this:
[FullName = $value] [id != $ObjectToCompare]
You can then simply use a validation feedback activity. Similar to this:
In the microflow where you commit your object: