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It's not explicit in the spec that a WebVTT cue identifier and a WebVTT region identifier are supposed to be unique. But they should be - otherwise the CSS selection via ::cue(#identifier) makes no sense.
Why does it not make sense? Selectors support duplicate IDs fine.
Browsers will deal in CSS and HTML with multiple id attributes having the same value, but it's not conformant, not in HTML [1] and not in CSS [2]. [1] http://www.w3.org/html/wg/drafts/html/master/dom.html#the-id-attribute [2] http://www.w3.org/TR/selectors/#id-selectors I don't see why WebVTT should be different. WebVTT cue identifiers should also be unique.
Fixed in commit: https://github.com/w3c/webvtt/commit/8787b2532d0902525c2db5a8b1297c9e40809ef0