Attempting to add a duplicate key raises an exception.
Keys are always added to the base dictionary. Keys in MergedDictionaries within the current ResourceDictionary are not checked for duplication, only the base dictionary is checked for duplicates.
Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0
.NET Framework Client Profile
Supported in: 4, 3.5 SP1
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.