I stopped making proposals a couple of years ago believing I could remember to withdraw those that included overlapping cards before I'd have a problem on my hands. A few consecutive trades where I had to adjust for duplicates I didn't have caused me to change my methods.
Now when I set up a proposal, I'll compare it side by side using the technology we all should have on computers and make sure there is no overlap in what I'm offering along with checking that I have the card physically before submitting it. I'll do the same thing on offers I've received.
It's lowered my issues with missing cards from my end considerably. I'm probably at 99% with that system, and the one time I recall it not working was actually me missing the physical confirmation step.
Follow my blog - I Identify as a Card Collector.
“Aslan didn't tell Pole what would happen. He only told her what to do. That fellow will be the death of us once he's up, I shouldn't wonder. But that doesn't let us off following the signs.” - Puddleglum in The Silver Chair by C. S. Lewis