Fragments behaviour: FragmentTransaction::replace() and reverse backStack operation

I call this method to go forward from AFrag to BFrag:

showFragment()
{ 
    FragmentTransaction fragmentTransaction = mFragmentMgr.beginTransaction();

    // Add fragment to the container ContentView 
    fragmentTransaction.replace(R.id.operation_fragments_frame, mBFrag, mBFrag.getTag());

    // Add FADE effect
    fragmentTransaction.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);   

    // Keep the transaction in the back stack so it will be reversed when backbutton is pressed
    fragmentTransaction.addToBackStack(null);

    // Commit transaction
    fragmentTransaction.commit();
}

It shows a new fragment (BFrag), replace the previous one (AFrag) and keep info about the transaction, so it can be reversed/undone automatically on back button pressed.

When back button is pressed everything looks fine, the previous fragment is shown (AFrag). But when I go forward again (AFrag -> BFrag) I got a "Fragment already added exception".

Didn't the reverse开发者_高级运维/undone operation remove the new fragment (BFrag)? Is this the expected behaviour?

That's weird because after this, I decided to set a check:

 if(mBFrag.isAdded()) 
 {
    fragmentTransaction.show(mBFrag);
 }
 else 
 {
   fragmentTransaction.replace(R.id.operation_fragments_frame, mBFrag, mBFrag.getTag());
 }

and stills, it gets into the else statement... and I get the exception.

Any insight on what am I doing wrong, please?

Thx.

---

have you tried to use an other method, like remove(), then do an add(). or anything similar? I saw on some other post that the replace() method does not always behave correctly.