Why are some contacts shown several times? Some of my contacts are on SIM card and some are on phone. I want to keep all at one place.  If I transfer them from SIM card to phone, am I going to have duplicate contacts or it is smart enough to figure out?

If I select the option "all contacts" then one contact is shown several time in the list, like one for wiber, waze, whatapps etc. Please help me to manage my contacts.
if your contacts are shown from different accounts (google &amp; non-google accounts), just disable one of the accounts to hide multiple contacts. 
If you have duplicate contacts on your Google account, simply go to Google Contacts website via desktop, and choose option "merge (or delete, I can't remember) multiple contacts". Sync your phone contacts and you're done.