Oracle 11gR2: ORA-01102: cannot mount database in EXCLUSIVE mode
ORA-01102 cannot mount database in EXCLUSIVE mode
  
  Cause: Some other instance has the database mounted exclusive or shared.
  Action: Shut down the other instance or mount in a compatible mode.


Find whether you have any running Oracle process.


$ ps -ef | grep smon


Kill the process then try to start again.

Since I dont have the procedure which you have followed to perform DB rename and DBID change I can not say whether there is problem with your procedure. 

I would recommend to follow Oracle Knowledge Document to perform such task. 
How to Change the DBID and the DBNAME by using NID