How to check if a User exists and build a workflow

So we have multiple Customer Contacts for each Customer. Since we need to send emails to the Customer Contacts, we have built a workflow on Contact Person which will create a user (non-licensed user) for every Contact Person added (We have pre conditions for this too).

A single Contact Person can be a part of multiple Customers. So when we add the same person we do not want the duplicate users to get created. So is these a way to check this? 

I tried IsNew(CurrentUser()) in my precondition but didnt work.