Ordinarily, it is an error to specify a key that already exists in a database. That is:

john   john@host1
john   john@host2

Here, instead of replacing the first with the second, the second john line produces an error. To allow replacement keys, use the -r switch with makemap. Generally, the -r and -o switches should be combined when updating a database with new information. (See also the editmap program, Section 5.2.)