If i have had a personal loan for $21,000 and paid it all back what is the chance that i could borrow 40,000?

That is totally dependent on the rest of your credit. In general if your credit score is 700 or better you won't get declined as long as your debt to income ratio is not high. A bank will look at all of your commitments and see how much disposable income you have. Anything under a 40% Debt to income ratio and a credit score of 700 or better and you should be good to go. Some banks will allow lower credit scores and ratios but will raise the interest rate because you are a higher risk.