diff --git a/codearchiver/storage.py b/codearchiver/storage.py
index ee56afd..bd7641c 100644
--- a/codearchiver/storage.py
+++ b/codearchiver/storage.py
@@ -45,11 +45,15 @@ class DirectoryStorage(Storage):
 
 	def put(self, filename, index = None):
 		self._ensure_directory()
+		#FIXME: Race condition
+		if os.path.exists((targetFilename := os.path.join(self._directory, os.path.basename(filename)))):
+			raise FileExistsError(f'{targetFilename} already exists')
 		_logger.info(f'Moving {filename} to {self._directory}')
 		shutil.move(filename, self._directory)
 		if not index:
 			return
 		indexFilename = os.path.join(self._directory, f'{filename}.codearchiver-index')
+		# No need to check for existence here thanks to the 'x' mode
 		_logger.info(f'Writing index for {filename} to {indexFilename}')
 		with open(indexFilename, 'x') as fp:
 			fp.write(index.serialise())