gulp command is throwing the "Ignoring duplicate element definition: undefined" error at the vulcanize step

[cherukumilli]

Steps to recreate the problem:

1. git clone https://github.com/googlecodelabs/psk-es2015.git

2. bower install && npm install

3. $ gulp

[11:02:33] Using gulpfile ~/training/code-labs/psk-es2015/gulpfile.js
[11:02:33] Starting 'clean'...
[11:02:33] Finished 'clean' after 5.44 ms
[11:02:33] Starting 'default'...
[11:02:33] Starting 'copy'...
[11:02:33] Starting 'styles'...
[11:02:34] styles all files 98 B
[11:02:34] Finished 'styles' after 901 ms
[11:02:35] copy all files 12.97 MB
[11:02:35] Finished 'copy' after 2.44 s
[11:02:35] Starting 'elements'...
[11:02:35] Starting 'js'...
[11:02:36] Finished 'elements' after 1.05 s
[11:02:42] Finished 'js' after 6.99 s
[11:02:42] Starting 'jshint'...
[11:02:43] Starting 'images'...
[11:02:44] Starting 'fonts'...
[11:02:44] Starting 'html'...
[11:02:44] Finished 'fonts' after 625 ms
[11:02:45] Finished 'jshint' after 3.24 s
[11:02:45] html all files 53.12 kB
[11:02:45] Finished 'html' after 1.86 s
[11:02:45] images all files 28.17 kB
[11:02:45] Finished 'images' after 2.8 s
[11:02:45] Starting 'vulcanize'...
Ignoring duplicate element definition: undefined
Ignoring duplicate element definition: undefined
[11:02:49] Finished 'vulcanize' after 3.74 s
[11:02:49] Starting 'rename-index'...
[11:02:49] Finished 'rename-index' after 1.78 ms
[11:02:49] Finished 'default' after 16 s

4. $ gulp serve

shows the following errors in the google chrome developer console

2015-12-15 11:03:28.673 /deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.
2015-12-15 11:03:28.801 polymer.html:366 Uncaught TypeError: Cannot read property 'length' of undefined
2015-12-15 11:03:28.837 app.js:66Uncaught TypeError: Cannot set property 'scrollTop' of null