It is not double-tagging, they are entirely separate features. One is the path of the river, and the other is the water-covered area of the river.

Tagging a name on a river area is always incorrect tagging. If Nominatim is indexing names on waterway=riverbank or natural=water + water=river, then that behavior is simply incorrect, and has been incorrect for over a decade.

False, both tagging systems were widely in use at the time I became involved, sometimes just one, sometimes just the other, and sometimes both. If Nominatim was failing to correctly handle the former but not the latter style tagging, then it was already failing to properly handle a significant percentage of the river area objects tagged worldwide when I became involved.

If this effort further exposed an issue with Nominatim that was already widely present, I would consider that a positive outcome.

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