If you sync your Google contacts between multiple devices or pull from multiple email accounts, you've probably got quite a few duplicate entires. Now your Google and Gmail contacts let you kill those dupes en masse with a single button.
Hit up your contacts from Gmail, or head to google.com/contacts, and hit the "Find duplicates" button in the lower-right area. You'll be provided with the list of contacts with at least two entries each, which you can view in expanded form, and then either merge together or kill off. Not that this is some new-fangled tool or an advanced feature, but I know at least a few Google user's primary annoyances with contacts can be salved, if not cured, with this little button.
One button to merge all duplicate contacts [Official Gmail Blog]