Stop a piece from capturing itself

shatrizualizer.html

@@ -306,6 +306,12 @@
 					renderedPromotion = whiteMove ? whitePieces[promotion] : blackPieces[promotion];
 				}
 
+				// Are we moving the piece onto itself?
+				if (startLetter === endLetter && startNumber === endNumber) {
+					alert('Start and end squares of the move cannot be identical');
+					return;
+				}
+
 				// Are we moving the piece from where it is located?
 				let startSquare = document.getElementById(startLetter + startNumber);
 				if (startSquare.childNodes.length === 0) {