This. Why is this still a discussion point? The book very clearly says, in plain English, if you buy two of the same weapon they become twin-linked. You can then add a third of the same weapon type, meaning you effectively have 1 twin-linked and 1 non-twin-linked weapon. If you buy 4 of the same weapon on your commander then you have 2 twin-linked weapons. It doesn't need FAQing because it's very clearly written in the book. They have put out a day zero FAQ and didn't mention it, so clearly it wasn't an issue they felt needed addressing (because, and I can't emphasise this enough, the codex clearly explains how the process works).