Handling duplicate items in a list

[giorgio93p]

Is your feature request related to a problem? Please describe.
It would be nice to have a mechanism to detect whether an item that is being added to a list already exists (checked off or not) in the same list (and maybe in other lists).

Describe alternatives you've considered
I can think of some alternatives that would help, but I don't know which would be easy to implement and in line with the project's goals.

• Show a warning when trying to add the duplicate item, with options to either proceed or use the existing item.
• Add the ability to search within a list.
• Prohibit duplicates (this might be too restrictive, so a configuration switch might be needed in this case)