This key is already associated with an element of this collection (Error 457)

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 (in Windows) or HELP (on the Macintosh).