Logback duplicates records

I have a problem with a logback configuration. Here is my logback.xml:

<configuration>

<appender name="EMS" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>../logs/ems.log</file>
    <append>true</append>
    <encoder>
        <pattern>[%d{ISO8601}] {%t} [%p] %c - %m%n</pattern>
    </encoder>
    <rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
        <fileNamePattern>../logs/ems.log.%i</fileNamePattern>
        <minIndex>1</minIndex>
        <maxIndex>99</maxIndex>
    </rollingPolicy>
    <triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
        <maxFileSize>50MB</maxFileSize>
    </triggeringPolicy>
</appender>

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
   <encoder>
     <pattern>[%d{ISO8601}] {%t} [%p] %c - %m%n</pattern>
   </encoder>
</appender>

<logger name="a.b.c" level="DEBUG">
  <appender-ref ref="EMS"/>
</logger>

<logger name="a.b.c.d.SomeServiceImpl" level="INFO">
    <appender-ref ref="EMS"/>
</logger>

<logger name="a.b.c.e.SomeFilter" level="INFO">
    <appender-ref ref="EMS"/>
</logger>

<root level="OFF">
    <appender-ref ref="EMS"/>
</root>

</configuration>

The problem is when I set the following line for the root logger:

<appender-ref ref="EMS"/>

in the ems.log file each line duplicates:

[2013-12-31 13:16:59,217] {[ACTIVE] ExecuteThread: '0' for queue: 'weblogic.kernel.Default (self-tuning)'} [DEBUG] a.b.c.f.CallerImpl - qweqweqwe
[2013-12-31 13:16:59,217] {[ACTIVE] ExecuteThread: '0' for queue: 'weblogic.kernel.Default (self-tuning)'} [DEBUG] a.b.c.f.CallerImpl - qweqweqwe

but when I set the following:

<appender-ref ref="STDOUT"/>

everything works as expected:

[2013-12-31 13:16:59,217] {[ACTIVE] ExecuteThread: '0' for queue: 'weblogic.kernel.Default (self-tuning)'} [DEBUG] a.b.c.f.CallerImpl - qweqweqwe

I've expected that as far as the root logger had the 'OFF' level, there should't have been problems...

Can anybody, please, assist?