fix: handle duplicate input

internal/services/auth/register_staff.go

@@ -9,6 +9,10 @@ import (
 )
 
 func (a *AuthSvc) RegisterStaff(spec authdomain.RegisterStaffReq) (string, error) {
+	_, err := a.staffRepo.GetStaffByEmail(spec.Email)
+	if err == nil {
+		return "", errors.New("this email address is already in use")
+	}
 	hashedPwd, err := HashPassword(spec.Password)
 	if err != nil {
 		return "", err