The default SPD Workflow lookup is limited to 1 comparison criteria of type FieldName = Value, and returns the first item that matches. What you need to do is match on two fields: Title and Author. This is not possible via this method.
You will have to fetch your items by using an HTTP call to access the REST API. This is ... complicated, especially the first time you try it. Here is a good primer:
In addition to the basic methodology listed here, you'll need to filter the results to bring back only the items that match Title and Author. Again, a short primer:
Your final URL will look something like this:
[%URL of Current Site%]/_api/lists/getByTitle('Your List Name')/items?$filter=(Author eq [%current item:Author%]) and (Title eq [%current item:Title%])&$select=ID