Then try to check if the member already exists or not, something like this would work,
AchimotaGCDb repo = new AchimotaGCDb();
var sexlist = new SelectList(new[] { "Male", "Female" });
ViewBag.SexList = sexlist;
if(repo.First(x => x.MemberId == member.MemberId) == null) {
repo.Insert(member);
} else {
}
There are other functions available as well, such as Contains, IndexOf etc, you can try them out as well, the overall logic remains the same — you search for a member with specific details and then if the user does not exist, you add him, otherwise you return; or show an error.