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Ethereum transaction isn't proof someone is alt account.
Sending tokens to one accumulating address is in 99.99% situations solid proof, but you have to be careful because tokens might end up in decentralized exchange and they use one wallet for many accounts.
Centralized exchange and casinos use separate addresses for different accounts, although some sites are rotating ethereum addy's and giving the same address to different accounts, but not at the same time(
I saw this only one time). As casino(most of them?
) doesn't accept ERC tokens, it is possible that connection made by ethereum transaction will give false positive result. And, also, there are various deals between people, giveaways etc...
I was looking for such guide as I'm new in reporting connected accounts, that's interesting.
There are many ways, look for social media accounts, bitcoin addresses, addresses which are part of the same wallet, referral links in signatures....