FTR, it's not just officewiki, and not just "*:echo:seen:message" (there's also "*:echo:seen:alert"). Based on this snapshot, it's happening at a rate of roughly 45000 times per 15 minutes, hence spamming logstash.

Change 552257 had a related patch set uploaded (by Mobrovac; owner: Mobrovac):
[operations/mediawiki-config@master] kask-echoseen: Do not report dupes

https://gerrit.wikimedia.org/r/552257

Change 552268 had a related patch set uploaded (by Mobrovac; owner: Mobrovac):
[mediawiki/core@master] HashBagOSTuff: Do not insert false values

https://gerrit.wikimedia.org/r/552268

Ok, I think I found the true culprit for these messages. The fix is in the above patch. The trick here is that it is not uncommon for an Echo's cache key to be missing from the cache (in this case, from both caches). But the way the CachedBagOStuff works is that if the in-memory cache doesn't return a value, it asks the backend for it and stores it in the in-memory one unconditionally. However, since there is a high chance that that value was false, that would create a paradox of sorts in the code whereby the in-memory bag would say it has the key, but would return false. The above patch corrects the in-memory cache behaviour so as not to store the value if it's set to false.

Change 552268 abandoned by Mobrovac:
HashBagOSTuff: Do not insert false values

Reason:
Not a good route

https://gerrit.wikimedia.org/r/552268

In T237143#5681605, @mobrovac wrote:

Ok, I think I found the true culprit for these messages. The fix is in the above patch. The trick here is that it is not uncommon for an Echo's cache key to be missing from the cache (in this case, from both caches). But the way the CachedBagOStuff works is that if the in-memory cache doesn't return a value, it asks the backend for it and stores it in the in-memory one unconditionally. However, since there is a high chance that that value was false, that would create a paradox of sorts in the code whereby the in-memory bag would say it has the key, but would return false. The above patch corrects the in-memory cache behaviour so as not to store the value if it's set to false.

Heh I was wrong here. CachedBagOStuff explicitly checks for $value === false && !$hash->hasKey( $key ), so the above patch was not a fix for this.

Change 552257 merged by jenkins-bot:
[operations/mediawiki-config@master] kask-echoseen: Do not report dupes

https://gerrit.wikimedia.org/r/552257

Mentioned in SAL (#wikimedia-operations) [2019-11-25T23:07:43Z] <urbanecm@deploy1001> Synchronized wmf-config/CommonSettings.php: SWAT: d71b0ab: kask-echoseen: Do not report dupes (T237143) (duration: 00m 53s)

This was deployed during SWAT. See: https://logstash.wikimedia.org/goto/a61eb70c51d26b11835e4bb4caadda0b

Done.

This duplicate get() was solved by @matmarex in 39be79c1b0d15e21e533237834939b17b71bbc31 (REL1_44+), so reportDupes may be removed in the config of kask-echoseen to re-activate this log and catch possible future issues.