In some cases (for example when doing rpm -Fvh)
rpm failes with dependencies and installs the same package twice.
This is an example: test 2,3,4 are the same package
the only difference is package revision
(you can use any package for this).

[root@eth0_extrenal noarch]# rpm -Uvh test-1.0-2.noarch.rpm 
[root@eth0_extrenal noarch]# rpm -q test
test-1.0-2
[root@eth0_extrenal noarch]# rpm -Fvh test-1.0-3.noarch.rpm
test-1.0-4.noarch.rpm 
findMatches: cannot read header at 0x3a8
[root@eth0_extrenal noarch]# rpm -Fvh test-1.0-3.noarch.rpm 
[root@eth0_extrenal noarch]# rpm -Fvh test-1.0-4.noarch.rpm 
[root@eth0_extrenal noarch]# rpm -Fvh test-1.0-*            
[root@eth0_extrenal noarch]# rpm -q test
test-1.0-3
test-1.0-4

# rpm -q rpm
rpm-4.0-0.59

See, test package now installed TWICE.
Now the only way to fix this is
rpm --rebuilddb