Okay, the error message may not be 100% accurate, but you get the idea.

Another thing to be aware of is that the --addsign option does not check for multiple identical signatures. Although it doesn't make much sense to do so, RPM will happily let you add the same signature as many times as you'd like:

#  rpm  --addsig  blather-7.9-1.i386.rpm

Enter  pass  phrase:  <passphrase> (Not echoed)



Pass  phrase  is  good.

blather-7.9-1.i386.rpm:

# rpm --addsig blather-7.9-1.i386.rpm

Enter pass phrase: <passphrase> (Not echoed)



Pass phrase is good.

blather-7.9-1.i386.rpm:

#  rpm  --addsig  blather-7.9-1.i386.rpm

Enter  pass  phrase:  <passphrase> (Not echoed)



Pass  phrase  is  good.

blather-7.9-1.i386.rpm:

#  rpm  --addsig  blather-7.9-1.i386.rpm

Enter  pass  phrase:  <passphrase> (Not echoed)



Pass phrase is good.

blather-7.9-1.i386.rpm:

#  rpm  --checksig blather-7.9-1.i386.rpm

blather-7.9-1.i386.rpm: size pgp pgp pgp pgp md5 OK

#

As you can see from --checksig's output, the package now has four identical signatures. Maybe this is the digital equivalent of pressing down extra-hard while writing your name.