Multiple versions installed?

[Marco]

Hi, I am experiencing a strange (to me at least) behavior when
upgrading packages. Basically, I have package foo, version 3.1,
release 12345:

$ rpm -qivp foo-3.1-12345.x86_64.rpm
...
Version     : 3.1
Release     : 12345
...

After I install this with

rpm -ivh foo-3.1-12345.x86_64.rpm

it correctly appears in the list of installed packages with rpm -qa.
However, when I upgrade to release 12347 using the command

rpm -Uvh foo-3.1-12347.x86_64.rpm

sometimes both versions then appear in the list of installed packages:

$ rpm -qa | grep foo
foo-3.1-12345
foo-3.1-12347

The strange thing is that it doesn't happen always; sometimes only the
last one shows up (as should be, I think).
Is it possible that I'm doing something wrong or some meta information
is missing from my packages?

Thanks in advance.