unchecked value also adding in arraylist

I have two activities A and B,In my Activity A i have one button user will click on that button and redirect to B

This is my Activity A

public class NewsActi extends AppCompatActivity{
    private Button txtvw;
    public final static int GO_FOR_SORTINGDATA = 909;
    private String fid;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.news_act);
        txtvw = (Button) findViewById(R.id.button);

        if(getIntent().getExtras()!=null)
        {
            fid=getIntent().getStringExtra("filterid");

        }
        else
        {
            System.out.println("NewsActi is null");
        }
        System.out.println("NewsActi"+fid);

        txtvw.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intentF = new Intent(NewsActi.this, MainActivity.class);
                if (fid != null) {
                    intentF.putExtra("updtfiltid", fid);
                } else {
                    intentF.putExtra("updtfiltid", "null");
                }
                startActivityForResult(intentF,GO_FOR_SORTINGDATA);

            }
        });
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == Activity.RESULT_OK) {
            switch (requestCode)
            {



                case GO_FOR_SORTINGDATA:
                    try {
                        fid=data.getStringExtra("filterid");



                        if(fid!=null)
                        {


                            System.out.println("FILTER IN PRODUCTLIST" + fid);


                        }


                    } catch (Exception e) {

                        e.printStackTrace();
                    }
                    break;
            }
        }
    }

}

Now In my Activity B i have listview with checkbox, and i am parsing data and display it. Now Issue is first when i go to B and select two items and then click on button and sending selected ids to Activity A,now again i am going to B and and uncheck one selected item,and check another item from list.but still previously selected item is getting when i click on button.

Activity B

 btnSelection.setOnClickListener(new Button.OnClickListener() {


                                    @Override
                                    public void onClick(View v) {

                                        String data = "";
                                        List<ModelUser> stList = ((MyCustomAdapter) mAdapter)
                                                .getStudentist();


                                        for (int i = 0; i < stList.size(); i++) {
                                            ModelUser singleStudent = stList.get(i);

                                            if (singleStudent.isSelected() == true) {

                                                data = data+singleStudent.getId() +",";
                                            }

                                        }


                                        System.out.println("my data"+data);
                                        if(fid.equals("null"))
                                        {
                                            System.out.println("if data"+data);
                                        }
                                        else
                                        {
                                            data=fid+data;
                                            System.out.println("else data"+data);
                                        }
                                        Intent intent=new Intent(MainActivity.this,NewsActi.class);
                                        intent.putExtra("filterid",data);
                                        setResult(RESULT_OK, intent);
                                        MainActivity.this.finish();
                                        Toast.makeText(MainActivity.this, data, Toast.LENGTH_LONG).show();

                                    }
                                });
                            } catch (JSONException e) {
                                e.printStackTrace();
                            }

                        }