backup and restore

I have two databases on the same mssql instance - testdb and mirrordb.

testdb has some data. mirrordb has no data.

I want to copy the data of testdb into mirrordb.

so I want to backup testdb and restore mirrordb from the backup file of testdb.

I tried :

BACKUP DATABASE "testdb" TO DISK='c:\mssql\backup\testdb.bkup' WITH NOFORMAT,INIT,STATS=10
restore DATABASE "mirrordb" from DISK='c:\mssql\backup\testdb.bkup' WITH recovery,STATS=10

but get the error :

Msg 3154, Level 16, State 4, Line 1

The backup set holds a backup of a database other than the existing 'mirrordb' database.

Msg 3013, Level 16, State 1, Line 1

RESTORE DATABASE is terminating abnormally.

can somebody tell me how to do this right?

appreciate the feedback.