Can a poster and publication have the same title? Can a poster and its related publication have the same title or do they have to be different?
Obviously you should check for yourself with whoever you attempt to publish the manuscript with, but I've never seen a journal state that the title of a publication must be unique, and I have publications which have appeared in some form as posters with the same title.

It actually makes some sense to keep the titles the same. Any buzz that you might have generated with the poster may increase readership of the article. If you have separate titles then potential readers wouldn't have the association.