Fragment already added IllegalStateException

I use this method on my container Activity to show a BFrag

public void showBFrag()
{
    // Start a new FragmentTransaction
    FragmentTransaction fragmentTransaction = mFragmentMgr.beginTransaction();

    if(mBFrag.isAdded())
    {
        Log.d(LOG_TAG, "Show() BFrag");
        fragmentTransaction.show(mBFrag);   
    }
    else
    {
        Log.d(LOG_TAG, "Replacing AFrag -> BFrag");
        fragmentTransaction.replace(R.id.operation_fragments_frame, mBFrag);
    }

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

    // Commit transaction
    fragmentTransaction.commit();        
}

I call it from my container Activity; for the first time:

• gets into the else statement and mBFrag replace mAFrag.

Then I press the back button:

• and the operation is reversed (mAFrag is shown but.. does mBFrag is removed?).

Then I go forward again by calling showBFrag() from the same Activity:

• and it gets AGAIN into the else statement. (so I can deduce that mBFrag is NOT ADDED)
• but I got a Fragment already added IllegalStateException... (so why it didn't get into the if statement instead?)

So:

1. Why is the isAdded() method not returning TRUE if I'm getting a Fragment already added IllegalStateException??
2. Does popBackStack operation completely remove previously added fragments?
3. What behaviour am I misunderstanding?

EDIT: Here is the complete info of the exception.

06-07 12:08:32.730: ERROR/AndroidRuntime(8576): java.lang.IllegalStateException: Fragment already added: BFrag{40b28158 id=0x7f0c0085}
06-07 12:08:32.730: ERROR/AndroidRuntime(8576):     at android.app.BackStackRecord.doAddOp(BackStackRecord.java:322)
06-07 12:08:32.730: ERROR/AndroidRuntime(8576):     at android.app.BackStackRecord.replace(BackStackRecord.java:360)
06-07 12:08:32.730: ERROR/AndroidRuntime(8576):     at android.app.BackStackRecord.replace(BackStackRecord.java:352)
06-07 12:08:32.730: ERROR/AndroidRuntime(8576):     at myPackageName.containerActivity.showBFrag() // This line: "fragmentTransaction.replace(R.id.operation_fragments_frame, mBFrag);"

In the end my workaround was to execute remove() of the previous fragment and add() the new one. Although that's what replace() method was meant to do.

But I am still guessing why replace() method didn't work properly in this case. It is really weird and I want to discard that it is because I am misunderstanding something or doing something wrong.