So you changed your simcard, and it got a number after the name, like MTS 01.
You try to rename it in the settings, only to get an error. “Save failed – Duplicate name for SIM card” This problem has at least three solutions. The first and the easiest one is to just insert the old SIM card and rename it to something else, freeing the name currently occupied by it. This is not the best solution, as obviously your phone still keeps the data for the old SIM card. But if you don’t have root, or you are too lazy or your hands grow out of your ass and you jut can’t do anything right, it’s the best solution. The next way is to do a factory reset of the phone using standard Android feature for that. Needless to say you’ll loose all your settings. And the last and best way is to gain root, and then locate telephony.db file at this location. As you can see, this file has siminfo section keeping the historic data about your sim card changes. I couldn’t find a decent sqlite editor for Android, so I just copied the file over to my PC, edited out the lines, and saved the file back to it’s location. Don’t forget to set permissions. From now on, you can name the SIM card anything you want.