That's up to you.
Each swap is run independently and you have complete control over what you choose to receive so if you wanted duplicates of the same item you could achieve that by selecting it in different swaps. There would be zero chance of your getting duplicates without asking for them.
I don't see a problem with listing the same thing across multiple swaps. Some players may be the same, but it is very unlikely that all of them would be. What may be a dud in one swap could be highly prized in another.
If you want to make seeds of the same plant available to two swaps, for instance, make two entries for it in your trade list. Then you can choose to dedicate 1 of the two entries available to each of the two swaps by checking the far left column under #4 for this swap and checking under the number for your other swap for the other one.
