Fix duplicate key exception when building user mapping data

Author: satano

src/MigrationTools.Clients.TfsObjectModel/Processors/TfsExportUsersForMappingProcessor.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -67,8 +67,13 @@ protected override void InternalExecute()
 
             usersToMap = usersToMap.Where(x => x.Source.FriendlyName != x.Target?.FriendlyName).ToList();
             Log.LogInformation("Filtered to {usersToMap} total viable mappings", usersToMap.Count);
-            Dictionary<string, string> usermappings = usersToMap.ToDictionary(x => x.Source.FriendlyName, x => x.Target?.FriendlyName);
-            System.IO.File.WriteAllText(CommonTools.UserMapping.Options.UserMappingFile, Newtonsoft.Json.JsonConvert.SerializeObject(usermappings, Formatting.Indented));
+            Dictionary<string, string> usermappings = [];
+            foreach (IdentityMapData userMapping in usersToMap)
+            {
+                // We cannot use ToDictionary(), because there can be multiple users with the same friendly name and so
+                // it would throw with duplicate key. This way we just overwrite the value – last item in source wins.
+                usermappings[userMapping.Source.FriendlyName] = userMapping.Target?.FriendlyName;
+            }
             System.IO.File.WriteAllText(CommonTools.UserMapping.Options.UserMappingFile, JsonConvert.SerializeObject(usermappings, Formatting.Indented));
             Log.LogInformation("Writen to: {LocalExportJsonFile}", CommonTools.UserMapping.Options.UserMappingFile);