properly overwrite, even on windows

src/magic_folder/downloader.py

@@ -448,7 +448,13 @@ def mark_not_conflicted(self, relpath, keep_path, rejected_paths):
             for p in rejected_paths
         ]
         if dest_path != src_path:
-            src_path.moveTo(dest_path)
+            try:
+                src_path.moveTo(dest_path)
+            except FileExistsError:
+                # at least on Windows, it's an error to write to a
+                # file that's already there ..
+                src_path.remove()
+                src_path.moveTo(dest_path)
         for p in del_paths:
             try:
                 p.remove()