I have a double light switch in my house (1 inside, 1 outside light) that will trip the main switch in the circuit breaker when both switches are turned off at the same time. Turning either one off individually works fine, and turning them both on at the same time also works, but if both lights are on and I hit both switches, it'll trip the main switch and I lose power in the whole house.
Has anyone seen this before, or can someone suggest how I can start to diagnose this? I don't understand why this could be happening.
Thanks in advance!
Edit: I'm in the UK - completely forgot about that!
Just to be specific here, the unit in question is something like this: image. The left hand switch is responsible for the outside light (and is the only switch for these lights), the right hand one is responsible for the inside lights. There is another switch for the inside lights, so this is a 3 way switch.
This is not a new installation, has never worked properly that I'm aware of, and the switch has never been replaced.