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>