util.HashHistory: push() => ignore duplicate entries

while the browser-based hashchange event won't trigger for changes to the same value, other internal logic could do this.

one such example is controller.Application:

        me.importApp(path).then(module => {
            module.onStart();

            // short delay to ensure Component Controllers are ready
            config.hash && setTimeout(() => HashHistory.push(config.hash), 5);
        });