Mod Meadows posted on a reddit post over on the RS3 subreddit regarding people owning multiple accounts and rigging the raffle. Here's their response, despite being really vague...
Hi guys,
I wanted to let you know we have implemented a change to the September Raffle. This year players entering with multiple accounts cannot hinder your chances of winning an item from the raffle.
This change will make it more fair for the vast majority of you who will enter the raffle on a single account.
Good luck with your entries!
Cheers,
Mod Meadows