If you want an already used idea again, first you need to look at the complete list of ideas and check the status of the idea. An idea needs to be in an Active state to be dragged to the dropbox to create a filing.
To check the status of the idea, Go on Options, and select on Active, Archived, and Deleted Matters so that all available ideas start showing on the interface.
Search for the idea, through the idea number or keyword search in titles or tags.
Once done, go to the idea and if it is in an archived or deleted state, make the idea active again. Here you can see that the required idea is in an archived state. I will now unarchive the idea and make it active. The idea is now ready to be used in another filing.