I had never heard of such things as duplicate list item id. For troubleshooting purpose, you can run the following SQL Query in your content database:
SELECT
Webs.FullUrl, AllLists.tp_Title, AllUserData.*
FROM
AllLists INNER JOIN
Webs ON AllLists.tp_WebId = Webs.Id INNER JOIN
AllUserData ON AllLists.tp_ID = AllUserData.tp_ListId
WHERE
(AllLists.tp_Title = N'Tasks')
Note that running this query directly in production system is not supported, please backup your SharePoint content database and restore it into the test system and
run the query in test system.
And you may shall change the WHERE clause in the query to filter with your site url (Webs.FullUrl) and task list name(AllLists.tp_Title)
Different lists of cause will have duplicate value in AllUserData.tp_ID field, but there won't be duplicates in the same list.
