Same players should never have to pay bb or sb two rounds in a row. Currently this happens if new player arrives between dealer and blind positions. Button should be moved before considering new players.

Also, on most sites i've played at new players are not dealt any cards before they post big blind. Arriving players can choose to put in amount equal to bb in their current seat or wait untin they are sitting in bb position. This is done to avoid people constantly sitting out and moving around trying to avoid blinds or gain better position.