Angular Tracking and Duplicates
ngRepeat uses $watchCollection to detect changes in the collection. When a change happens, ngRepeat then makes the corresponding changes to the DOM:
.When an item is added, a new instance of the template is added to the DOM.
.When an item is removed, its template instance is removed from the DOM.
.When items are reordered, their respective templates are reordered in the DOM.
.track by for any list that may include duplicate values.
.track by also speeds up list changes significantly.
.If you don't use track by in this case, you get the error: [ngRepeat:dupes]