This happens when the Value field for a pick list associated with a field is not unique, and two (or more) records are pulled back that match. This causes the system to not know which record is the correct one, and produces an error.
Create a new pick list that utilizes a unique field for the Value field.
In the example below, NetworkUserName is a unique field, but the pick list is still displaying by DisplayName.
This allows the system to search on the unique Value field NetworkUserName and eliminates duplicate conflicts.