It doesn't need to be as secure as say a real voting system, 10,000 votes gets a response, 100,000 gets a debate, 100,000 means that it has served it purpose, numbers beyond that are meaningless. No point wasting money securing something that is not that important, it's just a bit of fun.The first part seems to be the case - I just tried signing up with two different names but the same email address, one a couple of minutes after the other and they both went through. I also tried signing up with a foreign email address and that also went through and I received the email link to click; however, to be fair, the qualifying box to tick states that one must be either a British Citizen or a UK resident. I suppose this makes the petition about as as reliable as the Vote Leave campaign and only slightly less of a scam.
It does seem to be a waste of time in the first place as the end result of the petition is a foregone conclusion: another pointless debate.