modify the setPiece method to make it remove any piece that was already

there first

src/backend/Board.java

@@ -33,6 +33,15 @@ public class Board {
 	}
 
 	public void setPiece(boolean isWhite, PieceType type, int x, int y) {
+		// 1. remove any piece that was already in this cell
+		for (int i = 0; i < pieces.size(); i++) {
+			Piece p = pieces.get(i);
+			if(p.getX() == x && p.getY() == y) {
+				pieces.remove(i);
+				break;
+			}
+		}
+		// 2. add the new piece 
 		pieces.add(new Piece(x,y,type,isWhite));
 	}