The first thing you want to do is to export the contacts from the different sources into a CSV (Comma Separated Value) format. For things like Outlook, check under File and look for Import/Export. The same goes for most email clients out there.
The next step is to log into your Gmail account and then select Contacts. Under More, you’ll find Import. Select Import and the choose the CSV file you exported from your mail program and click on Import.
One of the things that most people will run into are the duplicate contacts or two separate contacts where one will contain the email address and the other a phone number.
Google solved this by having a feature called, “Find & merge duplicates…” To get to this, select Contacts while in Gmail and then click on More. In there you’ll find “Find & merge duplicates…” Once you select it, you will see the contacts that are duplicates. Just click on Merge to consolidate them. I went from 400+ contacts to about 125.
For more information, visit Google’s blog on this topic: http://gmailblog.blogspot.com/2009/12/one-button-to-merge-all-duplicate.html