The only problem with that is that they'd also need to know your username. If that site was asking for two specific items of data like an e-mail address before they allow you to test your password then I'd question it a bit more.
Of course there is the possibility that they could monitor your IP address on the site, and find out where else that IP address has come up and what e-mail address was being used with the other sites (bit of a long winded way around to get the information and as many people have dynamic IP addresses highly unlikely to work - but it's certainly possible!)


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