If your Google account is already added to your Mac System Preferences with contacts enabled, you need to disable the contacts switch for the best results. Read more about it at https://playaapps.zendesk.com/hc/en-us/articles/226488587.
Make sure that you've given this app access to your Mac Contacts. Otherwise, this app won't be able to sync your contacts. To make sure that this app has access to your contacts, go to your Mac System Preferences > Security & Privacy > Privacy > Contacts, and make sure that this app is enabled.
Before syncing with Google for the first time, go to https://contacts.google.com and use Google's tool for eliminating duplicates if needed.
If syncing for the first time, and you would like to treat either Google or Mac as the "Master", then perform a "Gmail Contacts -> Mac" sync or a "Mac Contacts -> Gmail" sync. Otherwise, perform a 2-way sync.
After your first sync, I would generally recommend using a 2-way sync.