running mvn compile twice without a clean causing duplicate classes

The current build breaks when `mvn compile` is run twice with no `mvn clean` between. I've not been able to figure out what is going wrong. I did find a hack that works where you add -proc:none to the javac compiler arguments and also add an old maven-processor-plugin to the pom, but that seems.. well, silly.

Here's the errors:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.2:compile (default-compile) on project soy: Compilation failure: Compilation failure:
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/shared/AutoValue_SoyAstCache_VersionedFile.java:[9,7] duplicate class: com.google.template.soy.shared.AutoValue_SoyAstCache_VersionedFile
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_ControlFlow_IfBlock.java:[7,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_ControlFlow_IfBlock
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_FieldRef.java:[8,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_FieldRef
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jssrc/internal/AutoValue_HelperFunctions_FieldAccessStrategy.java:[7,7] duplicate class: com.google.template.soy.jssrc.internal.AutoValue_HelperFunctions_FieldAccessStrategy
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/soytree/AutoValue_ForNode_RangeArgs.java:[9,7] duplicate class: com.google.template.soy.soytree.AutoValue_ForNode_RangeArgs
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/soytree/AutoValue_TemplateDelegateNode_DelTemplateKey.java:[7,7] duplicate class: com.google.template.soy.soytree.AutoValue_TemplateDelegateNode_DelTemplateKey
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/soytree/AutoValue_CallNode_DataAttribute.java:[8,7] duplicate class: com.google.template.soy.soytree.AutoValue_CallNode_DataAttribute
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_ConstructorRef.java:[10,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_ConstructorRef
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_MethodRef.java:[10,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_MethodRef
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/msgs/restricted/AutoValue_SoyMsgPart_Case.java:[8,7] duplicate class: com.google.template.soy.msgs.restricted.AutoValue_SoyMsgPart_Case
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/conformance/AutoValue_ConformanceInput.java:[10,7] duplicate class: com.google.template.soy.conformance.AutoValue_ConformanceInput
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_TypeInfo.java:[8,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_TypeInfo
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_ClassData.java:[8,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_ClassData
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_CompiledTemplateMetadata.java:[8,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_CompiledTemplateMetadata
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_DetachState_ReattachState.java:[8,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_DetachState_ReattachState
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_VariableSet_VarKey.java:[7,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_VariableSet_VarKey
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_VariableSet_SaveRestoreState.java:[7,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_VariableSet_SaveRestoreState
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_SoyNodeCompiler_CompiledRangeArgs.java:[8,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_SoyNodeCompiler_CompiledRangeArgs
[ERROR] /Users/jmhodges/src/github.com/google/closure-templates/target/generated-sources/annotations/com/google/template/soy/jbcsrc/AutoValue_LazyClosureCompiler_ParentCapture.java:[7,7] duplicate class: com.google.template.soy.jbcsrc.AutoValue_LazyClosureCompiler_ParentCapture
```