To merge two or more (up to 5) duplicate objects, you can simply go to the list and find them (sorting and/or filtering can help here), then select the duplicates and click the Merge button.
If there are any conflicting values, it will allow you to select the correct one.
After resolving the issues, click the merge button.
You will be taken to the newly merged object.
The new object will contain all of the contact information from each of the source objects combined together.