I am on Verizon. Try this. Not sure if it will work as I haven't needed to do this since going to Android with a MOTO Droid but it shouldn't do any damsge. On the S6:
Contacts > More > Settings > Import/Export contacts > Export
You should be able to move the newly created (VCF) vCards off the phone via a cable to another computer, create a backup, and then play with the VCF files with a spreadsheet, text editor or something else. Or put them into Gmail directly:
Launch a browser and go to:
https://contacts.google.com/u/0/preview/all
On the left, under your account name and "All Contacts", choose More > Import and in the newly opened dialog box select: CSV or VCard file.
On the Contacts page
https://contacts.google.com/u/0/preview/all there is an option to Find Duplicates.
The process sounds messier than it really is. The VCF and the CSV standards make this kind of contact migration between dissimilar email systems -- PC, Mac, BlackBerry, Android, iOS -- doable. In the past, I have merged multiple mail accounts, including BlackBerry, and opted to do cleanup, editing and rearrange/relabel fields in a spreadsheet to produce a clean CSV file. I backed up the spreadsheet at each step before making a significant global edit.
Once you have imported the VCF or CSV file into your Gmail, a few minutes later your work should show up in the Gmail app and Contacts.
HTH. YMMV. FWIW VZW Tier 2 should confirm the Export to VCF process.