diff -r 4508b8b6b45b -r c8810910c311 javaquery/demo-calculator-dynamic/src/main/java/org/apidesign/bck2brwsr/demo/calc/Calc.java
--- a/javaquery/demo-calculator-dynamic/src/main/java/org/apidesign/bck2brwsr/demo/calc/Calc.java	Mon Mar 25 14:15:09 2013 +0100
+++ b/javaquery/demo-calculator-dynamic/src/main/java/org/apidesign/bck2brwsr/demo/calc/Calc.java	Mon Mar 25 16:47:17 2013 +0100
@@ -74,7 +74,9 @@
             c.getDisplay()
         );
         c.setDisplay(newValue);
-        c.getHistory().add(newValue);
+        if (!c.getHistory().contains(newValue)) {
+            c.getHistory().add(newValue);
+        }
         c.setMemory(0);
     }