logback-android/src/main/java/ch/qos/logback/core/spi/AppenderAttachableImpl.java
public void addAppender(Appender<E> newAppender) {
    if (newAppender == null) {
      throw new IllegalArgumentException("Null argument disallowed");
    }
     appenderList.addIfAbsent(newAppender);
  }
public void addAppender(Appender<E> newAppender) {
    if (newAppender == null) {
      throw new IllegalArgumentException("Null argument disallowed");
    }
     appenderList.addIfAbsent(newAppender);
  }
Attach an appender. If the appender is already in the list in won't be
added again.