The customer has a custom multi valued lookup attribute on their Project Object. The resource that is selected for this field only shows once on the Project Properties screen, but when we look at this same field on the Project list, it shows multiple times. Why?
The root cause of the issue is that the Hidden Key in the Lookup is not unique. You will need to correct you custom query to have a unique ID. You need a query in a way that the Hidden Key return unique value.