TypeToTag shouldn't contain aliases.

SharpYaml/Serialization/AssemblyRegistry.cs

@@ -176,6 +176,9 @@ namespace SharpYaml.Serialization
 		    lock (lockCache)
 		    {
                 tagToType[tag] = new MappedType(type, alias);
+
+                // Only register types that are not aliases
+                if (!alias)
                     typeToTag[type] = tag;
             }
 		}