Print NavMap error only once for invalid NavMesh.

modules/navigation/nav_map.cpp

@@ -573,7 +573,7 @@ void NavMap::sync() {
 					connections[ek].push_back(new_connection);
 				} else {
 					// The edge is already connected with another edge, skip.
-					ERR_PRINT("Attempted to merge a navigation mesh triangle edge with another already-merged edge. This happens when the current `cell_size` is different from the one used to generate the navigation mesh. This will cause navigation problem.");
+					ERR_PRINT_ONCE("Attempted to merge a navigation mesh triangle edge with another already-merged edge. This happens when the current `cell_size` is different from the one used to generate the navigation mesh. This will cause navigation problems.");
 				}
 			}
 		}