Cache leaks memory through duplicate dependency strings

Description

Fixed a cache memory leak.

When an item is inserted with a cache dependency, an entry is added to the cache dependency map, but it does not check if the same entry is already there, which opens the possibility of having duplicates added and uses memory unnecessarily.

Multiple duplicates were observed in a memory dump from a customer site. It might have been triggered because the Optimizely Search & Navigation indexer loaded the same blocks over and over again in steps of the indexing.