When making requests to the server

I have a problem, which I consider easy, but due to my little experience in Android I can not solve.

You see, I'm developing an app that gets the data from a server, that is, I make requests to get the data to be displayed Ejm: list of products.

I have a main activity, which calls fragments by a viewpager, everything works well until I want to see a product, that is, when I tap on one and send me to the other activity and back to the activity again main, it seems that the content is duplicated or the request is made again and it adds again the same elements

This is the MainActivity code

public class MainActivity extends AppCompatActivity {
    ViewPager pager;
    TabLayout tabLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        pager= (ViewPager) findViewById(R.id.main_pager);
        tabLayout= (TabLayout) findViewById(R.id.tab_layout);

        setupViewPager();


    }

private void setupViewPager() {
            pager.setAdapter(new PagerAdapter(getSupportFragmentManager(), buildFragments()));
            tabLayout.setupWithViewPager(pager);

            tabLayout.getTabAt(0).setIcon(R.drawable.ic_posts);
            tabLayout.getTabAt(1).setIcon(R.drawable.ic_action_message);

        }

        private ArrayList<Fragment> buildFragments() {
            ArrayList<Fragment> fragments = new ArrayList<>();

            fragments.add(new PostsFragment());
            fragments.add(new MensajesFragment());

            return fragments;
        }
}

the requests are made in the fragment, first to the requests in the state onResume () but I change it to the state onStart () and I still have the same problem.

This is the code of the Fragment

public class PostsFragment extends Fragment  {
    private RecyclerView recycler;
    private PostsAdapter adapter;

    public static PostsFragment newInstance() {
        PostsFragment fragment = new PostsFragment();
        return fragment;
    }

    public PostsFragment() {
        // Required empty public constructor
    }


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        adapter = new PostsAdapter(getActivity());
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View root =  inflater.inflate(R.layout.fragment_posts, container, false);

        // Obtener el Recycler
        recycler = (RecyclerView) root.findViewById(R.id.posts_recycler);
        recycler.setHasFixedSize(true);

        final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());

        recycler.setLayoutManager(linearLayoutManager);
        recycler.setAdapter(adapter);

        return root;


    }

    @Override
    public void onStart() {
        super.onStart();

        Call<PostsResponse> c = ApiAdapter.getApiService().getPosts(ApiCons.API_KEY,"","DESC");
        c.enqueue(new Callback<PostsResponse>() {
            @Override
            public void onResponse(Call<PostsResponse> call, Response<PostsResponse> response) {
                adapter.agregarTodo(response.body().getPosts());
            }

            @Override
            public void onFailure(Call<PostsResponse> call, Throwable t) {
                Toast.makeText(getActivity(), t.getMessage(), Toast.LENGTH_LONG).show();
            }
        });
    }

    @Override
    public void onResume() {
        super.onResume();


    }

could help me with your experience to solve this problem. Thanks.