There are different situations where the Maps (Kartotherian) rendering code is confronted with duplicates. Before, it refused to render anything in most of these situations. As a first improvement we decided to show whatever comes first, see T306543. Further possible improvements:

User provides duplicate Q-ids

This is a (partially) new feature, see T305822: GeoPoints via QID. The user-provided GeoJSON might contain something like "ids": "Q1, Q1". For whatever reason. Maybe this list is generated by a template that forgot to consider duplicates.

Possible interpretations: Showing the exact same information twice is useless, no matter if it's a shape, point, or whatever. The only information is the fact that some id appears multiple times, and how often. While we could render this as a number or a note, it's very unlikely this is of any use. This is different when the GeoJSON asks for the same Q-id with different properties, e.g. different colors.

Proposal:

1. Make the list unique before doing anything else with it. In other words, make "ids": "Q1, Q1" behave identical to "ids": "Q1".
2. Check if it's possible to show the same Q-id twice with different properties. Discuss what should happen then.

User-provided SPARQL returns same Q-id twice

This is only for the existing feature where a SPARQL query returns nothing but Q-ids, which are then resolved and rendered as shapes.

Proposal:

1. As long as there is no different information assigned to the duplicate Q-ids (e.g. different colors), drop the duplicates, as above.
2. Discuss what should happen when the properties are different. It doesn't make much sense to render the same shape twice, with different colors.

User-provided SPARQL returns same coordinate twice

This is only when the query returns coordinates that are then rendered as points.

Proposal:

1. Drop exact duplicates where all relevant information is identical.
   • Optional: Consider only fields than make a difference on the map. This includes the Q-id, coordinates, title, description and image, but no other fields.
2. Discuss what should happen when two different Q-ids have the same coordinate. These will be rendered on top of each other. One is probably unreachable for the reader.

Item actually contains multiple coordinates

Example: https://www.wikidata.org/wiki/Q833129. While this is rare and often an error (typically a missing "preferred" or "deprecated" flag), it's sometimes valid and needs to be considered. A good example is a lake that is considered a single thing, but is described with multiple shapes.

Proposal:

1. As a next incremental step render all points, all the same way. Make sure this only happens when the coordinates are actually different.
2. Discuss if we want to render the points somehow different, e.g. with an extra note like "Great Sea (multiple places)" or "Great Sea (1 of 2 places)".