Replacing a single file with Replaces:

[Karolina Lindqvist <pgd-karolinali@algonet.se>]

In the policy manual, section 7.5.1 "Overwriting files in other packages" it 
says:
"However, if the overwriting package declares that it Replaces the one 
containing the file being overwritten, then dpkg will replace the file from 
the old package with that from the new. The file will no longer be listed as 
`owned' by the old package. "

I read that to mean that you can actually replace a single file from a 
package, in another package. But I can't get that to work. If I put 
"Replaces:" and the package name which I want to replace a file in, dpkg 
still aborts and says that the file exists in multiple packages. 

How to get this feature to work, to replace a single file in a package from 
within another package?

-- Karolina