Unexpected behavior from define-union-language -- the "two sublanguages" named in the error message are the same language

gistfile1.rkt

#lang racket
(require redex)

(define-language LBase
  (e (+ e e)
     number))

(define-extended-language L1 LBase
  (e ....
     (- e e)))

(define-extended-language L2 LBase
  (e ....
     (* e e)))

(define-union-language LMerge (one. L1) (two. L2))

#|
define-union-language: two sublanguages both contribute the non-terminal: one.e in:
  (one. L1)
  (one. L1)
|#