the following SQL would duplicate the redirects that end with '/', making a copy without a slash.

insert_join_redirection.sql

INSERT INTO wp_redirection_items(
	`url`,
	`regex`,
	`position`,
	`last_count`,
	`last_access`,
	`group_id`,
	`status`,
	`action_type`,
	`action_code`,
	`action_data`,
	`match_type`,
	`title`
)
SELECT
	SUBSTRING(items.url, 1, LENGTH(items.url)-1) AS url,
	items.regex,
	@pos := @pos + 1 AS postion,
	0 AS last_count,
	'0000-00-00 00:00:00' AS last_access,
	items.group_id,
	items.status,
	items.action_type,
	items.action_code,
	items.action_data,
	items.match_type,
	items.title
FROM wp_redirection_items AS items
JOIN (SELECT @pos := MAX(sub.position) FROM wp_redirection_items AS sub) r
LEFT JOIN wp_redirection_items AS matched ON
	matched.url = SUBSTRING(items.url, 1, LENGTH(items.url)-1)
	AND matched.regex = items.regex
	AND matched.group_id = items.group_id
	AND matched.status = items.status
	AND matched.action_type = items.action_type
	AND matched.action_code = items.action_code
	AND matched.match_type = items.match_type
WHERE
	items.group_id = 1
	AND items.url LIKE '/%-%/'
	AND items.regex = 0
	AND items.status = 'enabled'
	AND items.action_code = 301
	AND items.match_type = 'url'
	AND matched.id IS NULL