Assuming you are doing that in Nuxeo-Studio:

Start by creating 2 operations:

1. The first of type: “Document > Create”, let's call it CREATE_MY_DOC.

     You could supply here a document path as a parameter of your expected/existing document.
   

2. The second of type: “User Interface > Add Info Message”, let's call it NOTIFY_DOCUMENT_EXISTS.

     This is just an example, you could use a void operation instead.
   

Then create your “logic” operation of type: “Execution Flow > Run Operation”

Give this operation an ID of the following form: (condition?op1:op2) where if the “condition” is true, then “op1” is executed, otherwise “op2”

In the condition statement you can check if your document exists, for instance using NXQL:

@{(Session.query("SELECT * FROM Document WHERE ecm:path='MY_PATH").size()==0?"CREATE_MY_DOC":"NOTIFY_DOCUMENT_EXISTS")}

This should launch the appropriate operation creating the document if, and only if, it does not yet exist.