Conflict checks are something lawyers and firms do prior to establishing the lawyer client relationship. Because lawyers have a duty of confidentiality to their clients, a conflict could impact whether a lawyer could represent you. To avoid this they have to ensure they’ve never dealt with the opposing party of a case, for example in a marital, separation if one spouse retains a lawyer, the other spouse cannot be represented by that lawyer or anyone else at their firm.
You said never dealt with, what if the person had dealt with the firm a few years ago, or on a separate unrelated matter?
That would probably still be a conflict, and while the firm may be able to work around, it in most cases they’d likely insists that the conflicted party retain counsel elsewhere.
