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As follows
1. I send the first Porygon
2. You attach the item, I trade the other for it to evolve.
3. I send Porygon2 back for a spit back or something
4. Attach DD, you trade it back to me for spit back
5. I send Porygon-Z back
Result should be you have Modest Porygon and Modest Porygon-Z, and I have data on both.