Yes, Oppo A3s does have the dual rear camera. It is 13MP + 2MP dual camera at the rear.
No. Oppo A3s supports 2G, 3G & 4G GSM networks.
Yes, Oppo A3s does have the GPS support.
No. Oppo A3s does not have the fast charging support. However, the phone comes with 5V charger.