diff --git a/GameMap.py b/GameMap.py
index 15f6a14..35cbc09 100644
--- a/GameMap.py
+++ b/GameMap.py
@@ -20,6 +20,11 @@ class GameMap:
     def add_mark(self, x, y, player):
         if (x <= self.max_size_x and y <= self.max_size_y) and (x >= 0 and y >= 0):
             self.data[(x,y)] = player
+            if (x, y) not in self.data:
+                self.data[(x,y)] = player
+            else:
+                errorMessage = "The mark is already busy by the Player" + self.data[(x, y)].get_name()
+                raise KeyError(errorMessage)
         else:
             errorMessage = "The mark" + str((x,y)) + " is beyond preset max values, max_x - " + str(self.max_size_x) + ", max_y - " + str(self.max_size_y)
             raise IndexError(errorMessage)