The error message indicates that either the database file you are trying to attached has already been attached in sql express or there is data corruption in your database file(.mdf file) , thus the db file could not be attached. I would suggest you connect to your sql express server through management studio to make a verification (Note here that since Sql Express use customer instance, in management studio, you must login using the same account as the one you used in your application). If that db file has been attached, detach it and take another try.
If there is no such file attached already in your sql express, I would suggest you first attach that mdf file to your database. If you still get this error, it means that the mdf file is data corrupted (information in ldf file and mdf file does not match) and cannot be used anymore---You need to restore it then.
Hope my suggestion helps