I hope you don't call the person's former company before he signed the contract, you should not do that because he may not have resigned and told anyone about it, this could compromise him if he decides to pull out at the last minute. I would be really pissed if someone would do that to me and I would make sure tell everyone this sneaky behaviour on Glassdoor. Also I've resigned once but decided to take a month off in between so dates wouldn't match.