多个类定义attr属性重复的问题:Attribute "xxx" has already been defined

<? xml version = "1.0" encoding = "utf-8" ?>
< resources >
     < declare-styleable name= "PreferenceHeader" >
        <!-- Identifier value for the header. -->
        < attr name= "id" format = "integer"/>
        < attr name= "icon" format = "integer" />
        <!-- The fragment that is displayed when the user selects this item. -->
    </declare-styleable >
    < declare-styleable name= "Preference" >
        < attr name= "icon" format = "integer" />
        <!-- The key to store the Preference value. -->
        < attr name= "key" format = "string" />
    </declare-styleable >
</ resources >

<? xml version = "1.0" encoding = "utf-8" ?>
< resources >
     < attr name = "icon" format = "integer" />
     < declare-styleable name= "PreferenceHeader" >
        <!-- Identifier value for the header. -->
        < attr name= "id" format = "integer"/>
        < attr name= "icon" />
        <!-- The fragment that is displayed when the user selects this item. -->
    </declare-styleable >
    < declare-styleable name= "Preference" >
        < attr name= "icon"  />
        <!-- The key to store the Preference value. -->
        < attr name= "key" format = "string" />
    </declare-styleable >
</ resources >