We have a class like this:

public class Foo { 
   @BsonId 
   public ObjectId id;

   @Nullable 
   private String optionalString;

   @Nullable
   public String getOptionalString() {
       return optionalString; 
   } 
}

For the above class,  when building the class model, an exception in thrown in PropertyMetadata class, stating that the `Nullable` annotation already exists. The reason for throwing the exception in discussed in link below:

https://github.com/mongodb/mongo-java-driver/pull/405#discussion_r128712243

Would it be possible to allow above use case somehow? Simplest solution I can think of is to allow the same annotation if it equals the existing one.