I tried to turn on Merging Similar Issues by enabling a features “projects:similarity-view” and “projects:similarity-indexing”. The tab ‘Similar Issues’ has appeared, but for a whole week it has always been empty. What I am doing wrong? 
You need this in your sentry.conf.py as well:
SENTRY_SIMILARITY_INDEX_REDIS_CLUSTER = "default"
Please keep in mind that this feature executes a lot of scripts on Redis that has very poor performance characteristics, and you would likely benefit from configuring a dedicated Redis for this. You can configure redis clusters in config.yml.
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