diff -r af170d42b5b3 -r c8810910c311 javaquery/demo-calculator/src/main/java/org/apidesign/bck2brwsr/demo/calc/staticcompilation/Calc.java
--- a/javaquery/demo-calculator/src/main/java/org/apidesign/bck2brwsr/demo/calc/staticcompilation/Calc.java	Mon Mar 25 11:50:36 2013 +0100
+++ b/javaquery/demo-calculator/src/main/java/org/apidesign/bck2brwsr/demo/calc/staticcompilation/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);
     }