If you come across a duplicated contact you can easily merge it together. In order to do this, edit duplicated contact, click on a link Merge and then choose the other contact you would like it to merge with. Please note this is a PRO feature.
After merging, the duplicated contact will be removed and all its data will be copied to the other contact.
You can also operate on group of contacts. Just go to Contacts, choose as many as you need to, and then right click on them. You will see a few options: