Can't add a Google calendar to ical (calendar)

Actually, I'm trying to add my wife's google calendar to her calendar on our iMac running Yosemite. I'm following the instructions that Google has provided. I open Calendar, select preferences, select Accounts, click on Google, click add. I then enter her gmail email address and google password. I then get an error dialog: "Duplicate Account - The Google account <wife's email address> already exists. Any idea what is going on? Should be easy to do this. She uses this google calendar at work (where she has to use a PC). She would like to see it show up in her Calendar (and hopefully her iPhone).

Any help would be appreciated.

Thanks