So if two people are shooting in burst mode, and both have an overlap in their shooting burst of say 2 seconds with 5 images per second. Then the chances is alr

So if two people are shooting in burst mode, and both have an overlap in their shooting burst of say 2 seconds with 5 images per second. Then the chances is already (2 * 5)/1000 to land a picture in the same millisecond. You'd expect in 100 burst of pictures taken like this for one to be taken at the same millisecond. It's not that unlikely. What's more unlikely is that they selected the same shot and found out about it. This probably happens multiple times every day, just by virtue of the billions of people in the world taking a photo every day.