Use HashSets instead of .Distinct

And don't cast to array / list where unnecessary

OpenRA.Mods.D2k/Activities/SwallowActor.cs

@@ -76,7 +76,7 @@ namespace OpenRA.Mods.D2k.Activities
 			positionable.SetPosition(self, targetLocation);
 
 			var attackPosition = self.CenterPosition;
-			var affectedPlayers = targets.Select(x => x.Owner).Distinct().ToList();
+			var affectedPlayers = targets.Select(x => x.Owner).ToHashSet();
 			Game.Sound.Play(SoundType.World, swallow.Info.WormAttackSound, self.CenterPosition);
 
 			foreach (var player in affectedPlayers)