Your rider item list contains all items that artists have ever requested at your events. It's entirely possible to have doubles end up in there, due to someone making a spelling mistake or USING CAPITALS.
If you want to clean up your list you can follow these steps:
Head on over to your Launchpad
Click Settings > Program > Rider items
Look carefully through your list. See something that doesn't need to be there?
Click the garbage icon behind it
And that's it!
Be careful not to delete any items that are currently being used in your events.