It really depends on the situation, as there are any number of reasons why the decisions might (appear to) disagree, but I would always start by trying to reconcile them. What reasoning led to the previous decision? How does it differ from the reasoning I made more recently? Are the two truly in conflict? If appropriate, I would solicit perspectives from other site moderators as well, especially if the previous decision was not made by me.
One possible conclusion is that the two decisions aren’t really in conflict. In that case, I’d try my best to explain why I think the situations were different enough to have justified different outcomes. Of course, this is always a dialogue, and I would keep my mind open to other perspectives, both from other moderators and from the rest of the community.
Another possible conclusion is that they were both the right decision at the time, but something else has changed. Maybe the community has changed its stance, or perhaps the circumstances have shifted. This is sometimes just the nature of things—the past is a foreign country, as they say—but I’d do my best to articulate why I think a change in moderation policy is justified.
A third possibility is that the previous situation was, for some reason or another, just a bad call, and while that is lamentable, it does not justify making the same bad call twice. Sometimes people make mistakes, and sometimes those mistakes are unfortunately not obvious until much later. While the mistake cannot be undone, I would try to at least put things right to the extent that it is still possible, and I’d document my reasoning as best as I could to prevent such missteps from happening in the future.
Finally, it is of course possible that my recent decision was simply wrong, and the previous decision was the right one. This is always the most difficult conclusion to come to, but that is why it is so crucial to reflect—as objectively and impartially as possible—on the differences in circumstances and reasoning that led to the different outcomes. Making the wrong call is never a pleasant thing to accept, but refusing to accept that it was wrong only makes a bad situation worse, so I would do everything I could to fix the damage and make amends. If anything, the silver lining to such situations is that they may serve as important learning experiences, and I will always do my best to take their lessons to heart.