Although that's not really a good argument: (1) we don't know a lot about anything outside our solar system and (2) if this has happened a second time on earth it would be impossible for us to know since that thing would most likely have been destroyed by one of the first generation organisms.

I've seen a link on HN in the past where the author was questioning why we think it only happened once - and whether every puddle on the planet might have the early stages of life appearing, only we discount it because we assume it's contaminated by the rest of the biosphere on the planet leaking in. I feel like it was some researcher rather than some conspiracy theorist, but I don't know where the article was or what the context was to find it.