Syntax error duplicate characters on escaped unicode sequences

When I attempt to run "rosie test" on this file:

-- Test duplicate characters  
new_line_character = [\u000D / \u000A]
-- test new_line_character accepts "\u000D"

I get this error message:

test.rpl
Syntax error
        [parser]: duplicate characters in character list:
        in test.rpl:2:23: new_line_character = [\u000D / \u000A]

If you're wondering where I got these expressions from: https://blogs.msdn.microsoft.com/lucian/2010/04/19/vb-and-c-grammar-for-vs2010/ (I'm looking at the EBNF for C#.)

I'm wondering why 000D and 000A are considered duplicate characters? I have no doubt I've got the syntax wrong but I can't seem to see what it is I've gotten wrong.