Badly-defined behavior calling `put` reusing existing key.

The current behavior seems to be replacing the value of the key, but increases total_bytes without retiring the existing item.

I do not think this behavior is well defined. It causes total_bytes and available_bytes to diverge, and eventually triggers #15.

How unique do we expect the keys to be?

Shall we raise an exception if the key same key is used twice, or shall we consistently retire the existing key/value when the same key is reused?

Start by reproducing a repeated put with the same key and inspect how total_bytes and available_bytes are updated. Review issue #15 and compare the two proposed behaviors—rejecting duplicate keys or retiring the existing value—then establish which invariant and outcome should define done.

Written by the indexing model from the issue text.