add a missing failing test and a fix for it

src/System.CommandLine.Tests/ParsingValidationTests.cs

@@ -119,6 +119,34 @@ public void When_FromAmong_is_used_for_multiple_arguments_and_invalid_input_is_p
                   .Be(LocalizationResources.Instance.UnrecognizedArgument("not-key1", new[] { "key1", "key2" }));
         }
 
+        [Fact]
+        public void When_FromAmong_is_used_multiple_times_only_the_most_recently_provided_values_are_taken_into_account()
+        {
+            Argument<string> argument = new("key");
+            argument.AcceptOnlyFromAmong("key1");
+
+            var command = new Command("set")
+            {
+                argument
+            };
+
+            var result = command.Parse("set key2");
+
+            result.Errors
+              .Should()
+              .ContainSingle()
+              .Which
+              .Message
+              .Should()
+              .Be(LocalizationResources.Instance.UnrecognizedArgument("key2", new[] { "key1" }));
+
+            argument.AcceptOnlyFromAmong("key2");
+
+            result = command.Parse("set key2");
+
+            result.Errors.Should().BeEmpty();
+        }
+
         [Fact]
         public void When_FromAmong_is_used_for_multiple_arguments_and_invalid_input_is_provided_for_the_second_one_then_the_error_is_informative()
         {

src/System.CommandLine/Argument{T}.cs

@@ -1,9 +1,7 @@
 // Copyright (c) .NET Foundation and contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using System.Collections.Generic;
 using System.CommandLine.Binding;
-using System.CommandLine.Completions;
 using System.CommandLine.Parsing;
 using System.IO;
 
@@ -181,6 +179,7 @@ public Argument<T> AcceptOnlyFromAmong(params string[] values)
         {
             if (values is not null && values.Length > 0)
             {
+                Validators.Clear();
                 Validators.Add(UnrecognizedArgumentError);
                 CompletionSources.Clear();
                 CompletionSources.Add(values);