If you run the script twice for Centos5 errata and Centos6 errata, an errata which has the same ID for both versions wont be updated to also apply for the latter.
Example:
[CentOS-announce] CESA-2012:0451 Important CentOS 5 rpm Update
[CentOS-announce] CESA-2012:0451 Important CentOS 6 rpm Update
ID is 0451. If you run the script for Centos5 first, the errata will be skipped for Centos6.
We would need a way to update existing errata