why have I been billed twice on 4/28/15 for the webroot google?

I only see an expired keycode from way back on the account under the email you used to register on the community.  So you must have your new keycode registered under another email address.  If you got double-billed, then just call our customer service and they'll be able to get that sorted out for you.  Sorry for the hassle!
http://www.webroot.com/us/en/support/contact

what is the telephone number?

Here you go: 1-866-612-4268

If it is webroot I did call them and they said it was up to google webroot to reply.
And why would you question my being billed twice for the mobile update?

Oh, it's a purchase through the Google Play store for mobile?  Let me check and see if there's any other way for us to handle those.