In a content type we have the possibility to add the plugin typo "Availability" on a field.
This typo will check in Ajax if the content enter in the form field is not yet existing. To limit the check for the category where the content will be store, you have a parameter "Fields".
In this input you have to enter the category field name. Of course, the category field name must be in the form.
If your content type is used for multi languages, so you have one category for one language and another category for other language, you can add all category fields name in the input, separate by "||".
For example : art_catid_fr||art_catid_en
