How do you model your customer address data and what techinques are you using to ensure the quality of the data?
Things like deduplication algorithms, duplicate matches, making sure that packages and invoices can actually be delived and such? Esepcially in systems handling customers in multiple countries.
There are a number of vendors that provide address verification and normalization (converting multiple equivalent addresses into a standard form) as a service. Some of these vendors also offer the ability to figure out taxes at that address for invoicing purposes. Once you have the normalized form of the address, finding duplicates is just a matter of comparing entries (you might want to use a hash for speed). I'm hesitant to endorse a particular vendor of this software, or even list a few, on Stackoverflow...
There was a post that may help. It was posted a little while back: http://stackoverflow.com/questions/134956/how-do-you-perform-address-validation