This post claims that "according to folklorists" all Jacks are one and the same person: Jack and the beanstalk, Jack o'Lantern, Jack and Jill etc.
Is there a truth to it, in that there is a common, underlying motif that ties all the Jacks together? Or is it really one guy who ended up with a literal headlamp after a series of increasingly gruesome adventures? Or is it (as I suspect) that Jack is just a generic character name, like "Hans" in German Märchen or a John Doe?
The post in question served as excellent evidence that not everything found on reddit can be trusted!
I have never seen any folklorist who asserted that all Jacks are the same person. Such a fool may be out there, but I am unaware of it. Regardless, it certainly does not represent a consensus of any sort - not even three folklorists at a bar after a lot to drink are likely to believe this.
"Jack" is the common nickname for the name "John" - one of the more common names in Britain and the counterpart of Hans and Jon in Germany and Scandinavia respectively. The folk would not have imagined that all the Jacks were the same anymore than they would have thought that all the many Jacks in their village were actually the same person! Some village idiot may have thought that - but his name was probably Jack as well!