I have some duplicate things in my system. Not a big problem. When they come close enough together that I'm sure one is a duplicate, I remove it. The only possible problem is my reminder to water a certain plant once every two weeks. What if I had two of these, and was watering it every week, which would be too much for this plant? But I don't want to remove it and not water at all. Not a big deal -- I'll figure that out one way or another.
Suggestions: When you enter something that's written on a sticky note, just cross off the thing on the sticky note. I use a slash to mean "this has been copied elsewhere so I don't need it". That's like a check-mark to mean "done" or an X to mean "I've decided I don't need to do this". Or you can tear the corner of the sticky note or something.
Or, in your electronic system have a tag or flag meaning "this was copied from a paper source that still exists". If you go to enter it again and aren't sure if it will be a duplicate, you can check for that flag.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.