There's a certain small group (maybe 4 or 5) of users who I very strongly suspect (due to the combination of a very distinctive writing style, a shared affinity for particular very specific question types, common details in questions, and certain... reactions to on-site events) are the same person. I'm keeping those details somewhat vague both to avoid accusing anybody publicly (though I suspect many regular users already know who I'm thinking of), and to ensure answers are kept general rather than devolving into any discussion of whether I'm barking up the wrong tree with these particular users.
They have their share of closed questions, but most of their questions are broadly on-topic (and they probably have more than their share of questions making it to HNQ). Until now I've ignored it because I haven't seen any sign of them answering or commenting on each other's questions, or anything along those lines, and I don't see any obvious rule-breaking (i.e. abusing other users or posting offensive content). But the usage of multiple accounts strikes me as something that is forbidden on many websites, or allowed only with explicit disclosure, and I think there are good reasons for that. At least one of these accounts has previously received a ban on another stack exchange site I occasionally use, for consistently posting low quality questions. I'm not certain whether the other accounts continued to use that site during the ban.
My questions is: is there a policy regarding the usage of multiple accounts? If so, is it permitted? (If it is, I'll continue to ignore it as I have until this point.) If it isn't, what is the procedure for reporting (suspected) instances? Public accusations would obviously be inappropriate, and it doesn't seem to be the right usage of question or answer flags - but as far as I'm aware there's no way to flag a user.
additional information: I just noticed while browsing meta on another stack exchange site that several these users have recently been given suspensions there "for voting irregularities". This strongly suggests that, at least on the other stack, this set of multiple accounts has been used in a sock-puppet manner. I've flagged one of the accounts again, as I'm not clear whether this type of cross-site information is made available to moderators. If moderators find it helpful, I can re-flag the others too.
In fact, now I look deeper, various combinations of them are suspended for voting irregularities on several other stacks. I've re-flagged three accounts, to cover all of the stacks I could see immediately.