This is expected behavior for a one to many join, just as it would be in standard SQL. I would suggest skipping the join and creating a text property “ConcatBaskets” and set it using a Data Transform as shown below:
Cecil Howell, CSA | Senior Instructor | Pegasystems Inc.
I am testing on 7.1 so this may not apply but that is the error I get if I forget to prefix the property name with a dot. If the property is not optimized I only get a warning, this may be new in 7, 6 may require the property to be optimized. All you need to do is in the application explorer right click on ConcatBaskets and choose the optimize for reporting option and walk through the wizard.