Hi EG,
Hope my clarification below would help.
<blockquote>- If an item has “Japan” and “USA” in its extra-field
– If the user searches games released in Japan and/or in USA
– would the game be displayed twice in the search results ?</blockquote>
No, item will be displayed once in search result.
<blockquote>Secondly :
– if a user wants to search only the games released in USA AND in Japan
– is there an option to display only theses games and not the games only released in Japan for example ?</blockquote>
I guess that you mention this feature (Feature 1: add search mode for Multi select and searchable extra field types)
http://www.joomlart.com/documentation/joomla-component/ja-k2-filter#new-feature
In this case, you can configure search mode of Region field as Match-all option, so if user selects “USA” and “Japan” under Region field on search form, then search result only returns an item that matches all this search conditions.