Allow options to appear more than once

Unix has a strong tradition for scripting. In order to make it easier
to forward arguments to scripts, it's common practice to allow options
to appear more than once. The semantics are that the last one wins. So
this:

    $ tool -a this -b -a that

should be equivalent to

    $ tool -b -a that

Author: Immo Landwerth

src/System.CommandLine/README.md

@@ -134,6 +134,18 @@ is equivalent to:
 
     $ tool -a -m "hello"
 
+### Multiple occurrences
+
+Unix has a strong tradition for scripting. In order to make it easier to forward
+arguments to scripts, it's common practice to allow options to appear more than
+once. The semantics are that the last one wins. So this:
+
+    $ tool -a this -b -a that
+
+has the same effect as that:
+
+    $ tool -b -a that
+
 ### Parameters
 
 Parameters, sometimes also called non-option arguments, can be anywhere in the

src/System.CommandLine/src/System/CommandLine/ArgumentParser.cs

@@ -46,13 +46,12 @@ public bool TryParseOption<T>(string diagnosticName, IReadOnlyCollection<string>
                 return false;
             }
 
-            if (values.Count > 1)
-            {
-                var message = string.Format(Strings.OptionSpecifiedMultipleTimesFmt, diagnosticName);
-                throw new ArgumentSyntaxException(message);
-            }
+            // Please note that we don't verify that the option is only specified once.
+            // It's tradition on Unix to allow single options to occur more than once,
+            // with 'last once wins' semantics. This simplifies scripting because you
+            // easily combine arguments.
 
-            value = values.Single();
+            value = values.Last();
             return true;
         }
 

src/System.CommandLine/src/System/Strings.Designer.cs

@@ -150,9 +150,6 @@
   <data name="OptionsMustBeDefinedBeforeParameters" xml:space="preserve">
     <value>Options must be defined before any parameters.</value>
   </data>
-  <data name="OptionSpecifiedMultipleTimesFmt" xml:space="preserve">
-    <value>option {0} is specified multiple times</value>
-  </data>
   <data name="ResponseFileDoesNotExistFmt" xml:space="preserve">
     <value>Response file '{0}' doesn't exist.</value>
   </data>

src/System.CommandLine/src/System/Strings.resx

@@ -430,23 +430,6 @@ public void Option_Usage_Error_Conversion_CustomConverter()
             Assert.Equal("value 'abc' isn't valid for -o: invalid format", ex.Message);
         }
 
-        [Fact]
-        public void Option_Usage_Error_Duplicate()
-        {
-            var ex = Assert.Throws<ArgumentSyntaxException>(() =>
-            {
-                Parse("-a -b -a", syntax =>
-                {
-                    var arg1 = false;
-                    var arg2 = false;
-                    syntax.DefineOption("a", ref arg1, string.Empty);
-                    syntax.DefineOption("b", ref arg2, string.Empty);
-                });
-            });
-
-            Assert.Equal("option -a is specified multiple times", ex.Message);
-        }
-
         [Theory]
         [InlineData("-a")]
         [InlineData("-a:")]
@@ -511,6 +494,22 @@ public void Option_Usage_ViaModifer(string modifier)
             Assert.True(o);
         }
 
+        [Fact]
+        public void Option_Usage_LastOneWins()
+        {
+            var a = string.Empty;
+            var b = false;
+
+            Parse("-a x -b -a y -b:false", syntax =>
+            {
+                syntax.DefineOption("a", ref a, string.Empty);
+                syntax.DefineOption("b", ref b, string.Empty);
+            });
+
+            Assert.Equal("y", a);
+            Assert.False(b);
+        }
+
         [Fact]
         public void Option_Usage_Flag_DoesNotRequireValue()
         {