Are you sure you've got your figures correct?
If it's loaned out twice then it has been paid back early, obviously you wouldn't get say 5.5% interest if they paid it back after 6 months.
From Zopa, so if the loan is paid back early he will get his interest upto that point, and nothing going forwards :
"3. Can I repay my loan early if I like?
Yes. At Zopa, unlike at other loan providers, you can repay early, or in lump sums, at no extra cost. Because Zopa calculate the interest on your loan on a daily basis, this means you can save yourself lots of money if you choose to repay early.
For example, a £5000 loan over 36 months at 9.0% APR would cost a total of £5695 if you took the full 36 months to repay. But if you made your normal repayments and then repaid the whole of the remainder after 12 months, it'd cost a total of just £5373 - saving you £322. If you repaid in full after 18 months, the total cost would be £5509, and after 24 months it'd be £5609. So you can see the earlier you repay, the more you save. And Zopa don't charge a penny for this facility. However, you cannot settle or make an extra payment towards your loan whilst your monthly payment is in processing."
Yes he would quickly get his interest and capital back, but not the whole interest upto the end of the original loan date.
Realistically the calculation is simple,
(Return after 1 year) / (amount paid in)
But, after 1 year the return could have been reinvested into other loans, may have been turned over a few times etc
The funds also need to be discounted back to present value as £1 is worth less in 3 years than it is today.
I am not saying its a bad scheme but its certainly got some pitfalls that people not really sharp financially will be "duped" into thinking they are making larger returns relative to elsewhere then in reality they actually are.