Launch Main Activity only one time on Android

Use android:launchMode="singleTop" to make sure that a new instance of MainActivity isn't spawned when you press the back button. Spawning a new instance of MainActivity is not memory efficient.
Then, in the actual SettingsActivity you should override the home button to act like the back button:
if (id == android.R.id.home) {
    onBackPressed();
}
and you should display home as up, to allow up navigation:
this.getSupportActionBar().setDisplayHomeAsUpEnabled(true);

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