vars may produce duplicates

[michaelballantyne]

faster-minikanren/mk.scm

Lines 565 to 571 in dc5390a

	(define (vars term)
	(let rec ((term term) (acc '()))
	(cond
	((var? term) (cons term acc))
	((pair? term)
	(rec (cdr term) (rec (car term) acc)))
	(else (remove-duplicates acc)))))

When term is a pair such as (,x . ,x), the result would include two instances of the var. Not sure if this would lead to bad reification later or not.