My client is using this plugin and this is indeed a bug of sorts. I can explain in detail how it happens.
User A hasn't liked using the plugin yet. They click the "like" button in the popup, and then a Facebook.com Popup window appears. This window shows the Facebook page's icon and asks to confirm the like. Clicking "confirm" closes the window, but does not award any credits. The like goes through but the credits don't.
However, if you reload and the popup appears again, it will show the "like" button but with the checkmark meaning you have liked the linked page. At this point, if you click that button, it will "unlike" and if you click it again it will "like" the page and THEN award you the credits.
There seems to be an authorization problem somewhere keeping this plugin from being truly functional.