Avoid importing spawn points with the same location.

OpenRA.Mods.Common/UtilityCommands/Extensions.cs

@@ -9,6 +9,8 @@
  */
 #endregion
 
+using System;
+using System.Collections.Generic;
 using System.Data;
 using System.Text;
 
@@ -50,5 +52,15 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 			return result.ToString();
 		}
+
+		public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
+		{
+			var knownKeys = new HashSet<TKey>();
+			foreach (TSource element in source)
+			{
+				if (knownKeys.Add(keySelector(element)))
+					yield return element;
+			}
+		}
 	}
 }