I’m assuming you mean big ideas like inventing the television and not two people thinking “hey let’s get pizza!” at the same time.
The rational: If you consider that at any point in time, the average person was facing the same problems as all the other average people, it is not too unreasonable to think that two people could think of the same solution since they were facing the same problem and likely had similar life experiences and education.
The cynical: We’re not as unique as we think we are.
Cars have (or had, I don’t know if they’re still included) an electric lighter that you pushed in to heat up. It was basically a little metal ring that got really hot. So I think it probably is the heat.
I remember my friend also had a plasma lighter, that unsurprisingly used plasma to light things. It made a really scary noise in the wind and hurt like a motherfucker to touch. We were 18 and dumb.