flexjson.JSONException: Duplicate key "atc" when activate ProGuard

Question

I got the following JSONException when I run my app with ProGuard enabled. I tried different solutions found from other users but nothing seems to be working.

proguard-rules.pro:

-keep class flexjson.** {*;}
-keepclassmembers class flexjson.**{
*;
}
-keepattributes Signature, *Annotation*

-keep @flexjson.JSON class * {
<fields>;
}

JSONException

flexjson.JSONException: Duplicate key "atc"
        at flexjson.JSONTokener.putOnce(JSONTokener.java:498)
        at flexjson.JSONTokener.parseObject(JSONTokener.java:471)
        at flexjson.JSONTokener.nextValue(JSONTokener.java:357)
        at flexjson.JSONDeserializer.deserialize(JSONDeserializer.java:184)

I also tried to keep all classes including libraries but only works if I deactivate ProGuard. Thanks