class A declares multiple JSON fields

Since they are private fields there should not be any problem while creating json string

I don't think this statement is true, GSON looks up at the object's private fields when serializing, meaning all private fields of superclass are included, and when you have fields with same name it throws an error.

If there's any particular field you don't want to include you have to mark it with transient keyword, eg:

private transient BigDecimal tradeFeesPcy;

This is a bit late, but I ran into this exact same problem as well. The only thing was that I wasn't able to modify the superclass as that code wasn't mine. The way that I resolved this was by creating an exclusion strategy that skipped any field that had a field of the same name present in a superclass. Here is my code for that class:

public class SuperclassExclusionStrategy implements ExclusionStrategy
{
    public boolean shouldSkipClass(Class<?> arg0)
    {
        return false;
    }

    public boolean shouldSkipField(FieldAttributes fieldAttributes)
    {
        String fieldName = fieldAttributes.getName();
        Class<?> theClass = fieldAttributes.getDeclaringClass();

        return isFieldInSuperclass(theClass, fieldName);            
    }

    private boolean isFieldInSuperclass(Class<?> subclass, String fieldName)
    {
        Class<?> superclass = subclass.getSuperclass();
        Field field;

        while(superclass != null)
        {   
            field = getField(superclass, fieldName);

            if(field != null)
                return true;

            superclass = superclass.getSuperclass();
        }

        return false;
    }

    private Field getField(Class<?> theClass, String fieldName)
    {
        try
        {
            return theClass.getDeclaredField(fieldName);
        }
        catch(Exception e)
        {
            return null;
        }
    }
}

I then set the Serialization and Deserialization exclusion strategies in the builder as follows:

    builder.addDeserializationExclusionStrategy(new SuperclassExclusionStrategy());
    builder.addSerializationExclusionStrategy(new SuperclassExclusionStrategy());

Hopefully this helps someone!