diff --git a/object-file.c b/object-file.c
index 54a82a5f7a..440c5c6850 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1899,10 +1899,67 @@ static void write_object_file_prepare_literally(const struct git_hash_algo *algo
 	hash_object_body(algo, &c, buf, len, oid, hdr, hdrlen);
 }
 
+static int check_collision(const char *filename_a, const char *filename_b)
+{
+	char buf_a[4096], buf_b[4096];
+	int fd_a = -1, fd_b = -1;
+	int ret = 0;
+
+	fd_a = open(filename_a, O_RDONLY);
+	if (fd_a < 0) {
+		ret = error_errno(_("unable to open %s"), filename_a);
+		goto out;
+	}
+
+	fd_b = open(filename_b, O_RDONLY);
+	if (fd_b < 0) {
+		ret = error_errno(_("unable to open %s"), filename_b);
+		goto out;
+	}
+
+	while (1) {
+		ssize_t sz_a, sz_b;
+
+		sz_a = read_in_full(fd_a, buf_a, sizeof(buf_a));
+		if (sz_a < 0) {
+			ret = error_errno(_("unable to read %s"), filename_a);
+			goto out;
+		}
+
+		sz_b = read_in_full(fd_b, buf_b, sizeof(buf_b));
+		if (sz_b < 0) {
+			ret = error_errno(_("unable to read %s"), filename_b);
+			goto out;
+		}
+
+		if (sz_a != sz_b || memcmp(buf_a, buf_b, sz_a)) {
+			ret = error(_("files '%s' and '%s' differ in contents"),
+				    filename_a, filename_b);
+			goto out;
+		}
+
+		if (sz_a < sizeof(buf_a))
+			break;
+	}
+
+out:
+	if (fd_a > -1)
+		close(fd_a);
+	if (fd_b > -1)
+		close(fd_b);
+	return ret;
+}
+
 /*
  * Move the just written object into its final resting place.
  */
 int finalize_object_file(const char *tmpfile, const char *filename)
+{
+	return finalize_object_file_flags(tmpfile, filename, 0);
+}
+
+int finalize_object_file_flags(const char *tmpfile, const char *filename,
+			       enum finalize_object_file_flags flags)
 {
 	struct stat st;
 	int ret = 0;
@@ -1941,7 +1998,9 @@ int finalize_object_file(const char *tmpfile, const char *filename)
 			errno = saved_errno;
 			return error_errno(_("unable to write file %s"), filename);
 		}
-		/* FIXME!!! Collision check here ? */
+		if (!(flags & FOF_SKIP_COLLISION_CHECK) &&
+		    check_collision(tmpfile, filename))
+				return -1;
 		unlink_or_warn(tmpfile);
 	}
 
@@ -2195,7 +2254,8 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 			warning_errno(_("failed utime() on %s"), tmp_file.buf);
 	}
 
-	return finalize_object_file(tmp_file.buf, filename.buf);
+	return finalize_object_file_flags(tmp_file.buf, filename.buf,
+					  FOF_SKIP_COLLISION_CHECK);
 }
 
 static int freshen_loose_object(const struct object_id *oid)
@@ -2317,7 +2377,8 @@ int stream_loose_object(struct input_stream *in_stream, size_t len,
 		strbuf_release(&dir);
 	}
 
-	err = finalize_object_file(tmp_file.buf, filename.buf);
+	err = finalize_object_file_flags(tmp_file.buf, filename.buf,
+					 FOF_SKIP_COLLISION_CHECK);
 	if (!err && compat)
 		err = repo_add_loose_object_map(the_repository, oid, &compat_oid);
 cleanup:
diff --git a/object-file.h b/object-file.h
index d6414610f8..81b30d269c 100644
--- a/object-file.h
+++ b/object-file.h
@@ -117,7 +117,13 @@ int check_object_signature(struct repository *r, const struct object_id *oid,
  */
 int stream_object_signature(struct repository *r, const struct object_id *oid);
 
+enum finalize_object_file_flags {
+	FOF_SKIP_COLLISION_CHECK = 1,
+};
+
 int finalize_object_file(const char *tmpfile, const char *filename);
+int finalize_object_file_flags(const char *tmpfile, const char *filename,
+			       enum finalize_object_file_flags flags);
 
 /* Helper to check and "touch" a file */
 int check_and_freshen_file(const char *fn, int freshen);
diff --git a/tmp-objdir.c b/tmp-objdir.c
index a8e4553f27..9556f8fcdb 100644
--- a/tmp-objdir.c
+++ b/tmp-objdir.c
@@ -204,9 +204,11 @@ static int read_dir_paths(struct string_list *out, const char *path)
 	return 0;
 }
 
-static int migrate_paths(struct strbuf *src, struct strbuf *dst);
+static int migrate_paths(struct strbuf *src, struct strbuf *dst,
+			 enum finalize_object_file_flags flags);
 
-static int migrate_one(struct strbuf *src, struct strbuf *dst)
+static int migrate_one(struct strbuf *src, struct strbuf *dst,
+		       enum finalize_object_file_flags flags)
 {
 	struct stat st;
 
@@ -218,12 +220,18 @@ static int migrate_one(struct strbuf *src, struct strbuf *dst)
 				return -1;
 		} else if (errno != EEXIST)
 			return -1;
-		return migrate_paths(src, dst);
+		return migrate_paths(src, dst, flags);
 	}
-	return finalize_object_file(src->buf, dst->buf);
+	return finalize_object_file_flags(src->buf, dst->buf, flags);
 }
 
-static int migrate_paths(struct strbuf *src, struct strbuf *dst)
+static int is_loose_object_shard(const char *name)
+{
+	return strlen(name) == 2 && isxdigit(name[0]) && isxdigit(name[1]);
+}
+
+static int migrate_paths(struct strbuf *src, struct strbuf *dst,
+			 enum finalize_object_file_flags flags)
 {
 	size_t src_len = src->len, dst_len = dst->len;
 	struct string_list paths = STRING_LIST_INIT_DUP;
@@ -237,11 +245,15 @@ static int migrate_paths(struct strbuf *src, struct strbuf *dst)
 
 	for (i = 0; i < paths.nr; i++) {
 		const char *name = paths.items[i].string;
+		enum finalize_object_file_flags flags_copy = flags;
 
 		strbuf_addf(src, "/%s", name);
 		strbuf_addf(dst, "/%s", name);
 
-		ret |= migrate_one(src, dst);
+		if (is_loose_object_shard(name))
+			flags_copy |= FOF_SKIP_COLLISION_CHECK;
+
+		ret |= migrate_one(src, dst, flags_copy);
 
 		strbuf_setlen(src, src_len);
 		strbuf_setlen(dst, dst_len);
@@ -269,7 +281,7 @@ int tmp_objdir_migrate(struct tmp_objdir *t)
 	strbuf_addbuf(&src, &t->path);
 	strbuf_addstr(&dst, get_object_directory());
 
-	ret = migrate_paths(&src, &dst);
+	ret = migrate_paths(&src, &dst, 0);
 
 	strbuf_release(&src);
 	strbuf_release(&dst);