原因:

使用 Fragment 的状态保存，当系统内存不足，Fragment 的宿主 Activity 回收的时候，
Fragment 的实例并没有随之被回收。Activity 被系统回收时，会主动调用 onSaveInstance() 
方法来保存视图层（View Hierarchy），所以当 Activity 通过导航再次被重建时，
之前被实例化过的 Fragment 依然会出现在 Activity 中，此时的 FragmentTransaction 中的
相当于又再次 add 了 fragment 进去的，hide()和show()方法对之前保存的fragment已经
失效了。综上这些因素导致了多个Fragment重叠在一起。

解决方法：

第一种

1.给每个 Fragment 加一个 Tag
2.在 onCreate(Bundle savedInstanceState) 中判断 Bundle savedInstanceState 是否不为空
3.不为空则进行 find Tag，重新给几个 frament 赋值
这样子仍是对之前保存的 fragment 操作，成功解决了重叠的问题。

kotlin写法

class MainActivity : AppCompatActivity() {

    private var mHomeFragment: HomeFragment? = null
    private var mMessageFragment: MessageFragment? = null
    private var mMineFragment: MineFragment? = null
    private var mPreFragment: Fragment? = null
    private lateinit var mTransaction: FragmentTransaction


    override fun onCreate(savedInstanceState: Bundle?) {
        mTransaction = supportFragmentManager.beginTransaction()
        //savedInstanceState不为空, 状态恢复时, 重新查找一遍, 重新赋值
        savedInstanceState?.let {
            mHomeFragment = mHomeFragment ?: supportFragmentManager.findFragmentByTag("0") as HomeFragment
            mMessageFragment = mMessageFragment ?: supportFragmentManager.findFragmentByTag("1") as MessageFragment
            mMineFragment = mMineFragment ?: supportFragmentManager.findFragmentByTag("2") as MineFragment
        }
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initData()
    }

    private fun initData() {
        //选中监听
        mRgBottomMenu.setOnCheckedChangeListener { _, checkedId ->
            val transaction = supportFragmentManager.beginTransaction()
            mPreFragment?.let {
                transaction.hide(it)
            }
            when (checkedId) {
                R.id.mRbHome -> //home界面
                    mHomeFragment =
                        mHomeFragment?.apply { showFragment(transaction, this) }
                        ?: addFragment(transaction, 0) as HomeFragment

                R.id.mRbMessage ->//消息界面
                    mMessageFragment =
                        mMessageFragment?.apply { showFragment(transaction, this) }
                            ?: addFragment(transaction, 1) as MessageFragment

                R.id.mRbMine -> //我的界面
                    mMineFragment =
                        mMineFragment?.apply { showFragment(transaction, this) }
                            ?: addFragment(transaction, 2) as MineFragment
            }
            transaction.commitAllowingStateLoss()
        }
        mRbHome.isChecked = true
    }

    //显示已经存在的fragment
    private fun showFragment(transaction: FragmentTransaction, fragment: Fragment) {
        transaction.show(fragment)
        mPreFragment = fragment
        Log.d("test:: ", "show: ${fragment::class.java.simpleName} - $fragment")
    }

    //添加第一次创建实例的fragment
    private fun addFragment(transaction: FragmentTransaction, type: Int): Fragment {
        val fragment = when (type) {
            0 -> HomeFragment()
            1 -> MessageFragment()
            else -> MineFragment()
        }
        transaction.add(R.id.mFlRoot, fragment, "$type")
        mPreFragment = fragment
        Log.d("test:: ", "add: ${fragment::class.java.simpleName} - $fragment")
        return fragment
    }
}

java写法

public class MainActivity extends AppCompatActivity implements RadioGroup.OnCheckedChangeListener {
    private TextView mTvTitle;
    private LinearLayout mLlActionbar;
    private FrameLayout mFlRoot;
    private RadioButton mRbHome;
    private RadioButton mRbMessage;
    private RadioButton mRbMine;
    private RadioGroup mRgBottomMenu;


    private HomeFragment mHomeFragment;
    private MessageFragment mMessageFragment;
    private MineFragment mMineFragment;
    private Fragment mPreFragment;
    private FragmentManager mFragmentManager;


    @Override

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        mFragmentManager = getSupportFragmentManager();
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            mHomeFragment = (HomeFragment) mFragmentManager.findFragmentByTag("HomeFragment");
            mMessageFragment = (MessageFragment) mFragmentManager.findFragmentByTag("MessageFragment");
            mMineFragment = (MineFragment) mFragmentManager.findFragmentByTag("MineFragment");
        }
        setContentView(R.layout.activity_main);
        initView();
        initData();
        initListener();
    }
    //可要可不要
    @Override
    public void onAttachFragment(@NonNull Fragment fragment) {
//        if (mHomeFragment == null && fragment instanceof HomeFragment) {
//            mHomeFragment = (HomeFragment) fragment;
//        } else if (mMessageFragment == null && fragment instanceof MessageFragment) {
//            mMessageFragment = (MessageFragment) fragment;
//        } else if (mMineFragment == null && fragment instanceof MineFragment) {
//            mMineFragment = (MineFragment) fragment;
//        }
    }

    private void initView() {

        mTvTitle = findViewById(R.id.mTvTitle);
        mLlActionbar = findViewById(R.id.mLlActionbar);
        mFlRoot = findViewById(R.id.mFlRoot);
        mRbHome = findViewById(R.id.mRbHome);
        mRbMessage = findViewById(R.id.mRbMessage);
        mRbMine = findViewById(R.id.mRbMine);
        mRgBottomMenu = findViewById(R.id.mRgBottomMenu);
    }

    private void initData() {

    }

    private void initListener() {
        mRgBottomMenu.setOnCheckedChangeListener(this);
//        mRgBottomMenu.check(R.id.mRbHome);
        mRbHome.setChecked(true);
    }


    @Override
    public void onCheckedChanged(RadioGroup group, int checkedId) {
        FragmentTransaction transaction = mFragmentManager.beginTransaction();
        if (mPreFragment != null) {
            transaction.hide(mPreFragment);
        }
        switch (checkedId) {
            case R.id.mRbHome:
                if (mHomeFragment == null) {
                    mHomeFragment = new HomeFragment();
                    transaction.add(R.id.mFlRoot, mHomeFragment, "HomeFragment");
                } else {
                    transaction.show(mHomeFragment);
                }
                mPreFragment = mHomeFragment;
                break;
            case R.id.mRbMessage:
                if (mMessageFragment == null) {
                    mMessageFragment = new MessageFragment();
                    transaction.add(R.id.mFlRoot, mMessageFragment, "MessageFragment");
                } else {
                    transaction.show(mMessageFragment);
                }
                mPreFragment = mMessageFragment;
                break;
            case R.id.mRbMine:
                if (mMineFragment == null) {
                    mMineFragment = new MineFragment();
                    transaction.add(R.id.mFlRoot, mMineFragment, "MineFragment");
                } else {
                    transaction.show(mMineFragment);
                }
                mPreFragment = mMineFragment;
                break;
        }
        transaction.commitAllowingStateLoss();

    }
}

第二种

Activity 中的 onSaveInstanceState() 里面有一句super.onSaveInstanceState(outState);，
Google 对于这句话的解释是 “Always call the superclass so it can save the view hierarchy state”，
大概意思是“总是执行这句代码来调用父类去保存视图层的状态”。通过注释掉这句话，
这样主 Activity 因为种种原因被回收的时候就不会保存之前的 fragment state，
也可以成功解决重叠的问题。

@Override
    protected void onSaveInstanceState(@NonNull Bundle outState) {
//        super.onSaveInstanceState(outState);
    }

fragment 回退栈有关问题

通常使用fragmentManager.beginTransaction().addToBackStack(null);  

但是也经常会遇到说“某个fragment”已经被添加的错误，这往往是我们在对fragment进行返回处理时出现问题。这样的问题最常出现在我们点击界面进行跳转的时候。

一般情况，当我们点击导航栏进行界面切换之后，之前back stack中保存的fragment也就不再需要保留了。用下面的语句可以将back stack清空，同时也可避免上述错误的出现。

FragmentManager fragmentManager=getSupportFragmentManager();  
fragmentManager.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);