System.Data.SqlClient.SqlError: The backup set holds a backup of a database other 
than the existing 'xyz' database. (Microsoft.SqlServer.Express.Smo)

This is what I did 

open 
C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data

delete databasename.mdf
and
databasename_log.ldf

if the file is already in use then reboot the machine and try again. I think it should work