Protection against overwriting the key if it already exists in the DB

[RostyslavPin]

When you try to add the same key in db its overwriting.
Adding a check whether such a key already exists in the database.

[thodnev]

Nice catch!
But reading file to check whether it exists is overkill. This is better done with access(), stat(), or at worst with fopen(). And that logic should be placed in item_write() or xopen()