diff --git a/zack/git2blobs.py b/zack/git2blobs.py
index 8e8cde1..9145e1f 100755
--- a/zack/git2blobs.py
+++ b/zack/git2blobs.py
@@ -1,42 +1,45 @@
 #!/usr/bin/env python3
 
 """crawl a local Git repository and write all the blobs it contains to a
 Software Heritage like on-disk object storage
 
 """
 
 import dulwich
 import gzip
 import sys
 
 from dulwich.repo import Repo
 from pathlib import Path
 from tqdm import tqdm
 
 
 def store_blob(obj, objstorage_dir):
     sha1 = obj.id.decode('ascii')
     obj_path = objstorage_dir / sha1[0:2] / sha1[2:4] / sha1
+    if obj_path.exists():
+        return
+
     obj_path.parent.mkdir(parents=True, exist_ok=True)
     with gzip.open(obj_path, 'wb') as obj_f:
         for data in obj.chunked:
             obj_f.write(data)
 
 
 def main(repo_dir, objstorage_dir):
     repo = Repo(repo_dir)
     objs = repo.object_store
 
     objcount = len(list(objs))
     for sha1 in tqdm(objs, desc='writing blobs', unit='obj', total=objcount):
         obj = objs[sha1]
         if isinstance(obj, dulwich.objects.Blob):
             store_blob(obj, objstorage_dir)
 
 
 if __name__ == '__main__':
     try:
         main(sys.argv[1], Path(sys.argv[2]))
     except IndexError:
         print('Usage: git2objstorage REPO_DIR OBJSTORAGE_DIR')
         sys.exit(1)