Is an author allowed to use the same figure in different papers without citation? Authors A, B and C published paper [1] in journal X.
Author B created a figure for paper [1]. The figure is not essential and does not contain any important data. It is just a geometric ilustration.
Authors C and B are writing a new paper [2] for journal Y.


Can the same figure be used in paper [2] without any mention to paper [1]? Is it ethical? Does it violate copyrights of X? Is it considered plagiarism by Y? How about the situation X=Y? 

(Authors A, B and C are friends, so there is no problem for author A.)
Unfortunately there's no easy general answer to the copyright question, since it depends on the terms of the copyright assignment the authors signed with journal X, as well as the complexity and originality of the figure (you can't easily copyright an image of a triangle, you might have more difficultly with an image showing e.g. multiple stages of an algorithm). If the image was created programatically, then one possible solution might be to redraw it with slightly different data and new labelling.

It's also important to note that just citing the original source doesn't automatically make the copyright legal issues vanish. While the journal's home jurisdiction (or the authors') might grant you fair use rights/a fair trading defence, these each have their own restrictions, based on how much of the original the figure formed. Many academic publishers have a web portal allowing authors to request permissions to republish. These frequently (but not always) give reductions/waivers on fees to the original authors, or to articles submitted to their own journals.