Why it isn't a simple "more is better"
Both stores cap the fields they read for search: iOS gives 30 characters for the name, 30 for the subtitle, 100 for a hidden keyword field. Repeating a word already used in the name inside the keyword field burns characters you can't spare — it doesn't add a second match, it wastes the field.
Google Play works differently: the long description is indexed and has room for natural repetition, but stuffing it past what reads as normal copy risks a manual review flag rather than a ranking boost.
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