Actually, in retrospect, I'm not surprised at the cache_entry_name always being the same. It's an MD5 hash of the SQL query, and the query it's caching is unsurprisingly always going to be the same every time the cache entry is built/rebuilt/updated and thus the key's repetition is to be expected.
(Only a small handful of queries get cached this way, hence the limited sample size, but let's leave that topic off the table for now.)
This leads me to suspect a race condition of sorts, between the GC/expiry and creation of new records, perhaps in the busyness of rebuilding caches when the site has been idle for longer than cache and/or session expiration lengths.
As you observed, there appear to be no ill effects other than the errors logged, and I'm unsurprised that no further errors would appear, since the system will simply carry on with the next stage ... or the user will refresh the screen and things will carry on. (Okay, if they get a blank screen then that IS an ill effect, of course! but I don't recall seeing anyone saying that was a coexistent symptom.)
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