Gmail now lets you merge contacts. So if you have multiple entries for the same person cause they've sent you email from different addresses, you can merge them into a single contact. To do this go to your Contacts in Gmail and view your list of contacts. You can then check the multiple contacts that you want to merge into one. When you check a single contact in the list, it shows that contact information to the right. But as soon as you select more than you contact in the right you get a link to "Merge this x contacts...". Clicking this link will take you to a screen that shows the merged information. You can confirm/edit this information and then save it. Just like that your contacts are merged.