It's random but we never send double keys to our users. Furthermore in a short time we will improve our shop to include your steam game list (if you choose to link your steam account), by that way you will be sure to obtain an unique game each week.
Edit: some of games which are available for the current month are showed under the membership buttons.
My main concern with the premium service is exactly what @sirjaxxy brings to mind; we need more clarification on what we're actually buying into. A subscription service like this should be well detailed so that the buyer has as much information as possible prior to purchase. This type of premium subscription isn't quite as simple as say, Netflix, and is quite a bit more expensive. Thankfully there's no contract involved, unless OnGame is considering some type of twitch-like "partnership" program...
-cringe- ...