Fix dupe gridtree returns (#3517)

Robust.UnitTesting/Shared/Map/MapGridMap_Tests.cs

@@ -0,0 +1,31 @@
+using System.Linq;
+using NUnit.Framework;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
+using Robust.Shared.Maths;
+using Robust.UnitTesting.Server;
+
+namespace Robust.UnitTesting.Shared.Map;
+
+[TestFixture]
+public sealed class MapGridMap_Tests
+{
+    /// <summary>
+    /// Asserts FindGrids only returns the map once.
+    /// </summary>
+    [Test]
+    public void FindGrids()
+    {
+        var sim = RobustServerSimulation.NewSimulation().InitializeInstance();
+
+        var entManager = sim.Resolve<IEntityManager>();
+        var mapManager = sim.Resolve<IMapManager>();
+
+        var mapId = mapManager.CreateMap();
+        Assert.That(!mapManager.FindGridsIntersecting(mapId, Box2.UnitCentered).Any());
+
+        entManager.AddComponent<MapGridComponent>(mapManager.GetMapEntityId(mapId));
+        Assert.That(mapManager.FindGridsIntersecting(mapId, Box2.UnitCentered).Count() == 1);
+    }
+}