And physical petitions can't be manipulated????
Physical petitions can be verified because name, address and phone number are usually included. The bogus entries can be excluded. That is not the case with online petitions that require only a "name" or email address. I, personally, have no less than six email addresses and can create an unlimited number of additional email addresses, if necessary.
For these kinds of polls that require only a name and if one is devious enough to want to vote muliple times then all that is necessary is to vote, leave the site, empty the temporary internet files folder, return to the site and vote again. :-)