At the tax authority we could query that directly from a database.
But yes, the public sector suffers from countless different systems that have accumulated over the years. It happens because everything has to go out to a public bid and throughout the times it has been considered anti competitive to built things in-house.
More than that, the vendor winning the bid will happily use the opportunity to latch itself to the flow of taxpayer money. That's a big reason you end up with XML hell - the systems are designed to make future interop impossible without going through the vendor.
In fact, wasn't Denmark a case study of that? I remember there was someone on HN a while ago, IIRC working in public service of some country in Europe, who posted stories about their constant fighting with a vendor who intentionally makes it very hard to have any kind of integration and data exchange between various administrative branches.
But yes, the public sector suffers from countless different systems that have accumulated over the years. It happens because everything has to go out to a public bid and throughout the times it has been considered anti competitive to built things in-house.