Does navicular accessery bone result flat foot? If then than I have that only in one foot, thats on right foot but I have flat foot in both the foot.

No. Most people with flat feet have os navicularis (accessory) navicular bone. It does not cause flat feet. There are multiple causes for flat feet such as tight achilles tendons, spasm of the proneal muscles, tarsal coalition. Also posterior tibial tendon tears causes them.
Os navicular is. No what you are talking about is called a os navicularis and it has nothing to do with acquired pea planus or flat foot.