Only add event listener if it hasn't been added yet

common/static/js/capa/spec/formula_equation_preview_spec.js

@@ -115,6 +115,27 @@ describe("Formula Equation Preview", function () {
             ]);
         });
 
+        it('does not request again if the initial request has already been made', function () {
+            // jshint undef:false
+            expect(Problem.inputAjax.callCount).toEqual(1);
+
+            // Reset the spy in order to check calls again.
+            Problem.inputAjax.reset();
+
+            // Enabling the formulaEquationPreview again to see if this will
+            // reinitialize input request once again.
+            formulaEquationPreview.enable();
+
+            // This part may be asynchronous, so wait.
+            waitsFor(function () {
+                return !Problem.inputAjax.wasCalled;
+            }, "times out in case of AJAX call", 1000);
+
+            // Expect Problem.inputAjax was not called as input request was
+            // initialized before.
+            expect(Problem.inputAjax).not.toHaveBeenCalled();
+        });
+
         it('makes a request on user input', function () {
             Problem.inputAjax.reset();
             $('#input_THE_ID').val('user_input').trigger('input');

common/static/js/capa/src/formula_equation_preview.js

@@ -58,9 +58,17 @@ formulaEquationPreview.enable = function () {
             throttledRequest(inputData, this.value);
         };
 
+        if (!$this.data("inputInitialized")) {
+            // Hack alert: since this javascript file is loaded every time a
+            // problem with mathjax preview is loaded, we wrap this step in this
+            // condition to make sure we don't attach multiple event listeners
+            // per math input if multiple such problems are loaded on a page.
             $this.on("input", initializeRequest);
             // Ask for initial preview.
             initializeRequest.call(this);
+            // indicates that the initial preview is done for current $this!
+            $this.data("inputInitialized", true);
+        }
     }
 
     /**