Easiest way of finding and merging contacts is by using Google contacts. If you sync your device with Google account, you can easily merge duplicate contacts by using Gmail. For this, go to Google Contacts. This contact page lists all contacts you have saved in your Google account. Here, you can perform number of operations on these contacts. Find the “More” button at top and click on it. In the sub menu, select “Find & Merge Duplicate Contacts.”
In few seconds, Google will find all the duplicate contacts in your Google contacts. Now, if you would like to merge, just click on “Merge” button at the bottom.
After the merging done, you can again sync contacts with your device. And you have successfully merged all duplicate contacts of your Android device.
If you do not want use Google contacts method, you can use some apps to find and merge duplicate contacts.