java.lang.AssertionError: Duplicate type name

Hi, after updating j2objc to any newer version than 2.2, I get the following error without changing anything else in the codebase:
```
Error Output:
Exception in thread "main" java.lang.AssertionError: Duplicate type name: DopplRuntime
at com.google.devtools.j2objc.gen.ObjectiveCSourceFileGenerator.getOrderedGeneratedTypes(ObjectiveCSourceFileGenerator.java:160)
at com.google.devtools.j2objc.gen.ObjectiveCSourceFileGenerator.(ObjectiveCSourceFileGenerator.java:58)
at com.google.devtools.j2objc.gen.ObjectiveCHeaderGenerator.(ObjectiveCHeaderGenerator.java:52)
at com.google.devtools.j2objc.gen.ObjectiveCSegmentedHeaderGenerator.(ObjectiveCSegmentedHeaderGenerator.java:35)
at com.google.devtools.j2objc.gen.ObjectiveCSegmentedHeaderGenerator.generate(ObjectiveCSegmentedHeaderGenerator.java:39)
at com.google.devtools.j2objc.pipeline.TranslationProcessor.generateObjectiveCSource(TranslationProcessor.java:340)
at com.google.devtools.j2objc.pipeline.TranslationProcessor.processOutputs(TranslationProcessor.java:126)
at com.google.devtools.j2objc.pipeline.FileProcessor.processInputs(FileProcessor.java:69)
at com.google.devtools.j2objc.pipeline.TranslationProcessor.processInputs(TranslationProcessor.java:83)
at com.google.devtools.j2objc.J2ObjC.run(J2ObjC.java:127)
at com.google.devtools.j2objc.J2ObjC.main(J2ObjC.java:176)
```

We use some libraries from the [Doppl project](https://doppllib.github.io/), so `DopplRuntime` definetly comes from there.

My suspect from the stack trace is this change: 887f191eb62221ece06207d55c4913599c278604.

Does that change anyhow alter the order or method of dependency resolution, that's why j2objc _discovers_ `DopplRuntime` multiple times?

Any solution idea within the scope of j2objc as Doppl seems not to be updated anymore?

Thanks!