Do not filter out duplicate completions on non-Windows systems (#3454)

Author: JamesWTruher

PSReadLine/Completion.cs

@@ -10,6 +10,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Management.Automation;
 using System.Management.Automation.Runspaces;
 using Microsoft.PowerShell.Internal;
@@ -299,7 +300,8 @@ private CommandCompletion GetCompletions()
                     if (start < 0 || start > _singleton._buffer.Length) return null;
                     if (length < 0 || length > (_singleton._buffer.Length - start)) return null;
 
-                    if (_tabCompletions.CompletionMatches.Count > 1)
+                    // Only filter out duplicates on Windows.
+                    if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) && _tabCompletions.CompletionMatches.Count > 1)
                     {
                         // Filter out apparent duplicates
                         var hashSet = new HashSet<string>(StringComparer.OrdinalIgnoreCase);