I don't think it does at present.
It would not be difficult for the parser and validator to support it, or to represent the DISTINCT flag in the SqlNode AST, to add a 'boolean distinct' parameter to RexBuilder.makeOver.
It might be a bit more difficult to actually implement it, so one could run queries in Enumerable or other conventions. Maybe this could be done using a rewrite rule, converting a distinct-windowed-agg to a regular windowed-agg, but I can't quite think of the details.
Can you please log a JIRA case for this.