Fixed issue when bindingContext is bound more than once.

apps/tests/ui/bindingContext_testPage.ts

@@ -0,0 +1,34 @@
+import observableModule = require("data/observable");
+import pageModule = require("ui/page");
+
+class MainViewModel extends observableModule.Observable {
+    private _item: any;
+
+    constructor() {
+        super();
+
+        this.item = { Title: "Alabala" };
+    }
+
+    get item(): any {
+        return this._item;
+    }
+
+    set item(value: any) {
+        if (this._item !== value) {
+            this._item = value;
+            this.notifyPropertyChanged("item", value);
+        }
+    }
+
+    notifyPropertyChanged(propertyName: string, value: any) {
+        this.notify({ object: this, eventName: observableModule.Observable.propertyChangeEvent, propertyName: propertyName, value: value });
+    }
+}
+
+var viewModel = new MainViewModel();
+
+export function pageLoaded(args: observableModule.EventData) {
+    var page = <pageModule.Page>args.object;
+    page.bindingContext = viewModel;
+}