Remove duplicate test data in XOrDefault

We remove the files which would be duplicates on case-insensitive
filesystems.

This is necessary so that Git can work on Windows, whose filesystem is
infamously case-insensitive and a nightmare to work with in Git.

dev_scripts/find_duplicate_files_if_case_insensitive.py

@@ -0,0 +1,58 @@
+"""
+Find all the duplicate repository files on case-insensitive filesystems.
+
+This is in particular important for Windows so that we do not mess up Git there.
+
+You should run this script on a *case-sensitive* filesystem, *e.g.*, in Linux.
+"""
+
+import os.path
+import pathlib
+import sys
+import textwrap
+from typing import List, MutableMapping
+
+
+def main() -> int:
+    this_path = pathlib.Path(os.path.realpath(__file__))
+    repo_root = this_path.parent.parent
+
+    paths = []  # type: List[pathlib.Path]
+
+    for path in repo_root.iterdir():
+        if path.name == ".git":
+            continue
+
+        if path.is_file():
+            paths.append(path)
+        else:
+            assert path.is_dir()
+            paths.extend(
+                another_path
+                for another_path in path.glob("**/*")
+                if another_path.is_file()
+            )
+
+    path_map = dict()  # type: MutableMapping[str, List[pathlib.Path]]
+    for path in paths:
+        key = str(path).lower()
+        existing_paths = path_map.get(key, None)
+        if existing_paths is None:
+            path_map[key] = [path]
+        else:
+            existing_paths.append(path)
+
+    for key in sorted(path_map.keys()):
+        paths = path_map[key]
+        if len(paths) > 1:
+            paths_joined = "\n".join(
+                str(path.relative_to(repo_root)) for path in sorted(paths)
+            )
+            paths_joined_indented = textwrap.indent(paths_joined, "  ")
+            print(f"* Duplicates for {key}:\n{paths_joined_indented}")
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())