This Activity already has an action bar supplied by the window decor

I think you're developing for Android Lollipop, but anyway include this line:

<item name="windowActionBar">false</item> 

to your theme declaration inside of your app/src/main/res/values/styles.xml.

Also, if you're using AppCompatActivity support library of version 22.1 or greater, add this line:

<item name="windowNoTitle">true</item>

Your theme declaration may look like this after all these additions:

<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>
</style>

Another easy way is to make your theme a child of Theme.AppCompat.Light.NoActionBar like so:

<style name="NoActionBarTheme" parent="Theme.AppCompat.Light.NoActionBar">
     ...
</style>

Add single line android:theme="@style/AppTheme.NoActionBar" to activity in AndroidManifest and you've done.

AndroidManifest.xml:

<activity android:name=".activity.YourActivity"
          android:theme="@style/AppTheme.NoActionBar"><!-- ADD THIS LINE -->

styles.xml

<style name="AppTheme.NoActionBar">
        <item name="windowActionBar">false</item>
        <item name="windowNoTitle">true</item>
</style>

To use Toolbar as an Action Bar, first disable the decor-provided Action Bar.

The easiest way is to have your theme extend from

Theme.AppCompat.NoActionBar

(or its light variant).

Second, create a Toolbar instance, usually via your layout XML:

<android.support.v7.widget.Toolbar
    android:id=”@+id/my_awesome_toolbar”
    android:layout_height=”wrap_content”
    android:layout_width=”match_parent”
    android:minHeight=”?attr/actionBarSize”
    android:background=”?attr/colorPrimary” />

Then in your Activity or Fragment, set the Toolbar to act as your Action Bar:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.blah);

    Toolbar toolbar = (Toolbar) findViewById(R.id.my_awesome_toolbar);
    setSupportActionBar(toolbar);
}

This code worked for me.