Affects 5.6.6 for sure, and probably all 5.x versions after 5.6.0. Does not affect 6.x: fields are contributed differently and we already have a specific test for duplicate fields.
The problem is that, when there is a duplicate field contributed by a class bridge or class-level @Spatial annotation, we call documentFieldMetadata.getSourceType().getType() in org.hibernate.search.engine.metadata.impl.TypeMetadata#buildFieldMetadataMap, which is called in the constructor of TypeMetadata and thus before the back-reference is initialized, so it fails.
We should find another way to get the information than calling documentFieldMetadata.getSourceType().getType().
And yeah... there should be a test.