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Description of problem:
mount -o loop allows users to mount a filesystem already mounted with no warning. 

This seems dangerous. For example, when I run mount -o loop / it causes / to mount onto itself and immediate filesystem corruption.  

Version-Release number of selected component (if applicable):
Tested on RedHat 5, 6 and Fedora 17.  

How reproducible:
Always

Steps to Reproduce:
1. Mount a block device already mounted with the loop option:
mount -o loop <block device> <mount point>
  
Actual results:
A loopback device is created from the block device and mounted with no errors. 

Expected results:
The command fails with a warning that the filesystem is already mounted. 

Additional info:

The mount(8) command does not play such role in the system -- it's
kernel (filesystem driver) business to define what is allowed. The 
behavior is filesystem specific -- for example ext4 allows to use MMP
(multi-mount-protection, since e2fsprogs 1.42) to avoid this kind of
problems.

Seems like not a bug. Sorry.