Description of problem:

The command "cp -Z root:object_r:tmp_t file-a file-b" does not set the context 
of file-b if it already exists.  If the file does not exist then it operates 
correctly.

This is because the "-Z" option of cp sets the default context for new files 
that are created.  If a file exists then cp does not unlink and recreate it, 
but instead just truncates it.

One potential solution would be to unlink and recreate the file.  My solution 
has been to apply the create context when truncating the file (patch 
attached).  If you don't like my patch then let me know what you would like 
and I'll re-write it.