Restore a database with a different name on the same server
It's really simple to solve. You are trying to override the original files you need to you use the move option like this:
restore database &lt;Your-Database&gt; from disk ='&lt;backup-file-path&gt;' 
with move 'mdf_file_name' to '&lt;new-path&gt;',
move 'log_file_name' to '&lt;new-path&gt;'

By the way to discover the file name from backup use this command:
restore filelistonly from disk ='&lt;backup-file-path&gt;'