Move warning message to EndProcessing so it only shows up once (#9385)

src/System.Management.Automation/engine/ExperimentalFeature/EnableDisableExperimentalFeatureCommand.cs

@@ -30,6 +30,14 @@ namespace Microsoft.PowerShell.Commands
         /// </summary>
         [Parameter]
         public ConfigScope Scope { get; set; } = ConfigScope.CurrentUser;
+
+        /// <summary>
+        /// EndProcessing method.
+        /// </summary>
+        protected override void EndProcessing()
+        {
+            WriteWarning(ExperimentalFeatureStrings.ExperimentalFeaturePending);
+        }
     }
 
     /// <summary>
@@ -87,8 +95,6 @@ namespace Microsoft.PowerShell.Commands
                 cmdlet.WriteError(new ErrorRecord(new ItemNotFoundException(errMsg), "ItemNotFoundException", ErrorCategory.ObjectNotFound, name));
                 return;
             }
-
-            cmdlet.WriteWarning(ExperimentalFeatureStrings.ExperimentalFeaturePending);
         }
     }
 

test/powershell/engine/ExperimentalFeature/EnableDisable-ExperimentalFeature.Tests.ps1

@@ -94,4 +94,14 @@ Describe "Enable-ExperimentalFeature and Disable-ExperimentalFeature tests" -tag
         & $cmdlet ExpTest.FeatureOne -WarningVariable warning -WarningAction SilentlyContinue
         $warning | Should -Not -BeNullOrEmpty -Because "A warning message is always given indicating restart is required"
     }
+
+    It "Multiple features enabled will only output one warning message for <cmdlet>" -TestCases @(
+        @{ cmdlet = "Enable-ExperimentalFeature" },
+        @{ cmdlet = "Disable-Experimentalfeature" }
+    ) {
+        param ($cmdlet)
+
+        Get-ExperimentalFeature | & $cmdlet -WarningAction SilentlyContinue -WarningVariable warning
+        $warning | Should -HaveCount 1
+    }
 }