diff --git a/framework-dotnet/src/lib/MBS.Core/Collections/DuplicateKeyException.cs b/framework-dotnet/src/lib/MBS.Core/Collections/DuplicateKeyException.cs
new file mode 100644
index 0000000..7b2ab6b
--- /dev/null
+++ b/framework-dotnet/src/lib/MBS.Core/Collections/DuplicateKeyException.cs
@@ -0,0 +1,24 @@
+using System;
+
+namespace MBS.Core.Collections;
+
+
+/// <summary>
+/// The exception thrown when an item with the same key has already been added.
+/// </summary>
+public class DuplicateKeyException : ArgumentException
+{
+
+	public DuplicateKeyException() : base("An item with the same key has already been added.")
+	{
+	}
+	public DuplicateKeyException(Exception? innerException) : base("An item with the same key has already been added.", innerException)
+	{
+	}
+
+	public DuplicateKeyException(string? message) : base(message) { }
+	public DuplicateKeyException(string? message, Exception? innerException) : base(message, innerException) { }
+	public DuplicateKeyException(string? message, string? paramName) : base(message, paramName) { }
+	public DuplicateKeyException(string? message, string? paramName, Exception? innerException) : base(message, paramName, innerException) { }
+
+}