Add unit test

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ReduceNesting.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
@@ -566,5 +567,19 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				}
 			}
 		}
+
+		private static string DuplicateReturnTest(IDictionary<int, string> dict)
+		{
+			string value;
+			lock (dict)
+			{
+				if (!dict.TryGetValue(1, out value))
+				{
+					value = "test";
+					dict.Add(1, value);
+				}
+			}
+			return value;
+		}
 	}
 }