Of course it's to do with BrExit.
The whole goal of BrExit was to limit freedom of movement of people, goods and services, in other words, to "take back control".
The double taxation will happen if the seller does not properly declare that tax has already been paid, OR if you use a third party mail forwarding service like AddressPal.
In using a third-party service like that, you are doing as @NIR says, and having a product delivered to the UK, then subsequently exported to Ireland, almost like it's being sold a second time.
There is no friction at all when in one EU country and buying from another EU country - you pay whatever tax is due when you buy, and that's it, it's delivered to your door.
It's when you buy from or sell to a third country, be it China, the UK, the US or wherever, that the extra paperwork and the friction comes into play.
Hattons though, for example, are VAT registered in some EU countries, so in theory, once they actually declare that the VAT has been paid and don't forget to put a sticker on it (LOL @murphaph), it should arrrive while you're making your tea, no problem.
Although, who the Hell drinks tea nowadays?