As per my understanding, you want to check whether user is already exists before creating profile for that particular user.
Before decision try to retrieve user where first name and last name equals to the value given by data entry.
After retrieving it, in decision add condition like this
$user!=empty --→ if this is false then create profile object else show the page which you want to show.
Retrieve User Object:
Use union operation in list activity and use condition to check if it’s false then allow to create a profile
To add to the above answer – do note that using a first and last name to search for existing users is generally a bad idea, as there might be many users with the same name. It's recommended to add something unique to the user to make sure you actually have the right person (e.g. e-mail address, though that leaves people open to using different e-mail addresses)