system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Are there any other known twin wands besides Harry's and Voldemort's wands?
gpt
Due to the nature of wandmaking, there are probably quite a lot of sibling wands. Many wand cores are made from magical animal hairs, and it would make sense to harvest more than one hair at a time. Even with Dragon Heartstring wands it would make sense that you could get multiple cores from a single dragon's heart. 

Wandmaking is hardly industrialized, but just looking at how many wands Olivander has in his shop it seems clear that he probably doesn't make wands to order. Building up the stock of wands that we see in the background of his shop (and, although we don't see it, the store of wand parts he has to make more wands) doesn't make sense if he only receives one hair per thestral or unicorn that the core is harvested from. Wands are presumably affordable enough for wizards and witches to buy when necessary, and that (reasonably low) price doesn't mesh with the idea of harvesting only one wand core from each creature that can create them.

The reason Harry and Voldemort's wands were notable for their relationship was the nature of the animal that donated the core material. Phoenixes are fairly powerful magical beings, and Fawkes seems to be an extraordinary example of a phoenix. Add to that that phoenix feathers are fairly rare and that Fawkes only donated two feathers to be made into wand cores and those wands are almost guaranteed to be special.

Let's also not discount the potential effect that Harry and Voldemort's relationship itself adds to the situation. Wands are tools to channel the natural magical energy of witches and wizards, not the source of the magic itself. Harry had a piece of Voldemort's soul in him, and was presumably drawing some magical energy from that soul fragment. Voldemort's magical energy (from Voldemort) interacting with his own other magical energy (from Harry) is bound to create some weird magical situations, and even more so that both of these are being focused through very similar wands and that weirdness is going to be extra weird (such as what happened at the Gaunt graveyard during the Triwizard tournament).