Two files resolve to the same identifier (note: Identifier), because identifiers strip the extension (note: Plugin) and the type suffix (3 passages, no note), or because two notes (note: Note) declare the same id. The first in canonical order is kept.
Check (note: Check) E-ID-DUP, severity (note: Severity) error (29 passages, no note) by default, documented with a before and after example in the check pages. The severity can be overridden per project (note: Project) or per repository (note: Knowledge repository).