Uhh, here's an explanation to save you some time:
Let's say I lend you item ID 1000
It turns into ID, lets say, 1039.
I have one in my lending box thing that's, lets say, 1040.
Item ID 1000 disappears, and ID 1039 has limits.
Whenever the person gives it back, it checks to see if the other person has 1040, and if the borrower has 1039, then combines them and sends them to the lender.
There is no logical way to "duplicate" the item, because it checks to see if both of the new IDs are available then turns it back to the original item ID.
This is the reason why jagex can't do ALL of the items at the same time, and only has SOME items lendable.