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1. Was Roobet.io ever used by Roobet casino before the decided to settle for Roobet.com?
This domain was only registered 1 year ago on a whois basis, and I don't see any archive sites have recorded any gambling service content since it was registered. In conclusion, the com extension is registered first.
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2. If not, why not buy off that other domain (roobet.io) so that phishing scammers don't buy it and take advantage?
I tested the search engine with a fresh browser history and browsing cookies, didn't find that domain on the first page of search results this time. There are no signs of phishing using this domain yet.