How to copy database user on same database

If you want to copy a database user (including all the grants etc..), you should execute the following query. Each query will return you an executable SQL command. All returnings together will build a complete SQL script that will create a new user:
 
If any of the above has no output, you'll get an exception similar to this:

Then do a search and replace on the output to change the username and execute the script.