feat: name doubled destinations in exports

A platform list that declares one path for two files fails one of the
two checks on every install. The export report lists each declaration
with its hash and the emulators whose profile reads that file.

scripts/export_native.py

@@ -36,6 +36,7 @@ from artifacts import file_lock, write_bytes_atomic, write_if_changed, write_tex
 import yaml
 
 from common import (
+    build_zip_contents_index,
     platform_arg,
     list_registered_platforms,
     load_data_dir_registry,
@@ -50,6 +51,7 @@ from common import (
 from exporter import discover_exporters
 from exporter.baseline import HASH_FIELDS, _hash_values, _single_size, build_native_model
 from exporter.candidates import removal_candidates
+from exporter.doubled import doubled_destinations
 from truth import generate_platform_truth
 
 DEFAULT_CACHE = ".cache/upstream-native"
@@ -436,6 +438,24 @@ def refresh_cache(
     return True, [f"{len(wanted)} file(s) fetched"]
 
 
+@functools.cache
+def _collection(db_path: str) -> tuple[dict, dict]:
+    """The database and its archive index, read once for every platform."""
+    db = load_database(db_path)
+    return db, build_zip_contents_index(db)
+
+
+def doubled_in(scraped: dict | None, platforms_dir: str) -> list[dict]:
+    """The destinations the platform's own list declares for two files."""
+    if not scraped:
+        return []
+    db, zip_contents = _collection(TRUTH_DB)
+    return doubled_destinations(
+        scraped, load_emulator_profiles("emulators"), db, zip_contents,
+        load_data_dir_registry(platforms_dir),
+    )
+
+
 def _count(number: int, noun: str, plural: str = "") -> str:
     """"1 hash", "2 hashes": a count with its noun in the right number."""
     plural = plural or noun + ("es" if noun.endswith(("sh", "ch", "s", "x")) else "s")
@@ -764,6 +784,8 @@ def export_platform(
         systems, scraped, load_emulator_profiles("emulators"),
         load_data_dir_registry(platforms_dir),
     )
+    # Reported, never rewritten: both declarations stay as the platform wrote them.
+    doubled = doubled_in(scraped, platforms_dir)
     messages.append(summary)
     if unwritten:
         messages.append("not in the file: " + ", ".join(unwritten))
@@ -774,6 +796,11 @@ def export_platform(
             + (f" ({open_question} undetermined)" if open_question else "")
             + ", listed in the report"
         )
+    if doubled:
+        messages.append(
+            f"{_count(len(doubled), 'destination')} declared twice for two files"
+            ", listed in the report"
+        )
     messages.extend(f"withheld: {reason}" for reason in withheld)
     for heading, labels in (
         ("hash updated", applied),
@@ -807,6 +834,7 @@ def export_platform(
         "withheld": list(withheld),
         "found_by_content": report.labels,
         "removal_candidates": candidates,
+        "doubled": doubled,
         "invalid": issues,
     })
     return True, messages