In January, Gmail's Product Manager Todd Jackson said that Gmail will include a contact deduplicator. "Jackson says a de-duper is on the way and that contacts will have more of a presence both in your inbox and in conversations."
11 months later, Gmail added a feature that detects and merges duplicate contacts. "To clean up your contact list in one fell swoop, just click the Find duplicates button in the contact manager, review the merge suggestions (and uncheck any suggestions you don't want merged), and hit the Merge button." If you have many contacts, it's a good idea to consolidate them and to have a single contact for each person.
Other popular webmail services included this feature a long time ago: Yahoo Mail and Hotmail added tools for cleaning up duplicate contacts in 2007.