Don't allow switching into a Pokemon that's already in the field

PkmnLib.Dynamic/Models/Battle.cs

@@ -143,7 +143,7 @@ public interface IBattle : IScriptSource, IDeepCloneable, IDisposable
     /// <summary>
     /// Volatile scripts are scripts that are not permanent and can be removed by other scripts.
     /// </summary>
-    public IScriptSet Volatile { get; }
+    IScriptSet Volatile { get; }
 
     /// <summary>
     /// Gets the current weather of the battle. If no weather is present, this returns null.
@@ -341,6 +341,16 @@ public class BattleImpl : ScriptSource, IBattle
             if (preventMove)
                 return false;
         }
+        else if (choice is ISwitchChoice switchChoice)
+        {
+            if (!switchChoice.SwitchTo.IsUsable)
+                return false;
+            // Can't switch to a Pokémon already on the field
+            if (switchChoice.SwitchTo.BattleData is { IsOnBattlefield: true })
+                return false;
+            if (switchChoice.SwitchTo == switchChoice.User)
+                return false;
+        }
 
         return true;