Hi Genna!
It sounds like an onSubmit Catalog Client Script should do the trick. If you are using Service Portal / Employee Center/... as opposed to the native Service Catalog UI you'll want to do this with 2 of the same scripts onChange of the last_name and first_name variables, because the Service Portal asynchronous structure does not allow the submit to be aborted, and leave it up to users to complete/update the variables out of order. In any event you need to use a GlideAjax call to a Script Include. Here is an excellent guide if you're not familiar with how to do this:
https://www.servicenow.com/community/developer-articles/glideajax-example-cheat-sheet-updated/ta-p/2...
This goes a bit beyond what you need as it shows how to return more than one value, and in your case you'll just be returning a 'true' or 'false' or whatever with a return false; if it exists - or in an onChange script, clear one or both of the mandatory variables so that the request cannot be submitted. Your SI will do a GlideRecord query on the user table checking the first_name and last_name values passed in from the client script, and return one value if a (active?) record is found, or a different value if it is not found, then the callback function in the Client Script will evaluate that returned value to determine how to proceed. Give the scripts a shot, and let me know if you get stuck. Of course 2 people could submit a request with the same names and you would still wind up with duplicates if the first request is not fulfilled before the second one is created. You could also add a Business rule to the sys_user table to prevent a record from being added with the same first and last name fields as an existing (active) record, then the fulfiller would reject the second request in this event. Happy coding!
