That depends.

What was the command you used?

Either you will have two backups in that file or you will have one backup.

If there is only one backup in the file, then you overwrote the previous backup and you would not be able to restore to that backup.

If there are two backups in the file, then you can restore to either of the backups as long as the bak does not get damaged.

Then there is the whole other issue of whether the backups are corrupt or not.

To restore from multiple backups in a file (backup sets), you could do something like this:

RESTORE DATABASE AdventureWorks2012

FROM DISK = 'Z:\SQLServerBackups\AdventureWorks2012.bak'

WITH FILE = 9

RECOVERY;

The file option specifies the backup set within the file to use.