How to add a new user to the already existing Group and Memebership in JPP ?

Issue

• For User Creation, Assigning group and membership we wrote below piece of code:

        try {
            OrganizationService service = (OrganizationService) PortalContainer
                    .getInstance().getComponentInstanceOfType(
                            OrganizationService.class);

            User user = service.getUserHandler().createUserInstance("2943656544");
            user.setFirstName("DM Pvt.Ltd");
            user.setDisplayName("DM Pvt.Ltd");
            user.setEmail("d.ma@abc.com");
            user.setPassword("welcome1");
            log.info(user.getFirstName() + " : " + user.getDisplayName()
                    + " : " + user.getEmail() + ":" + user.getPassword());
            service.getUserHandler().createUser(user, true);
            log.info("User Created");

        GroupHandler groupHandler_ = service.getGroupHandler();
            Group group1 = groupHandler_.createGroupInstance();
            group1.setGroupName("/platform/users/myGroup");
            groupHandler_.addChild(null, group1, true);
            log.info("Group Created");

            MembershipHandler membershipHandler_ = service
                    .getMembershipHandler();

            MembershipTypeHandler mtHandler_ = service
                    .getMembershipTypeHandler();
            MembershipType mt = mtHandler_.createMembershipTypeInstance();
            mt.setName("myMembership");
            mtHandler_.createMembershipType(mt, true);

            log.info("Memebrship Cretaed");

            membershipHandler_.linkMembership(user, group1, mt, true);

            log.info("Assign membership");

        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            RequestLifeCycle.end();
        }

    }

Here i can successfully create user but, i want to assign existing group and membership type to that particular user.

With the above code, i get an error stating ,

Error when creating Membership: org.picketlink.idm.common.exception.IdentityException: Cannot create new relationship name: LNTEBG

this is because i already have a membership with name "myMembership" and also group with name "/platform/users/myGroup"