Visual Basic for Applications Reference
This key is already associated with an element of this collection (Error 457)
See Also Specifics
A key is a string specified in the Add method that uniquely identifies a specific member of a collection. This error has the following cause and solution:
You specified a key for a collection member that already identifies another member of the collection.
Choose a different key for this member.
For additional information, select the item in question and press F1.