People using London's cycle hire scheme are being charged an access fee for every key they hold, even if they use just one to release a bicycle.
One rider said he used one of his three keys over three days and was expecting a total £3 charge, but was billed £9.
The cyclist, identified only as Laurence, said he has since unregistered the two additional keys.
Transport for London said its website advises users they would be "charged against every key".
A spokeswoman explained that all keys registered under one cardholder will be charged the access fee but the usage charge for riding the bicycle will only apply to the key that released the bicycle.
About 29,000 people have signed up for Barclays Cycle Hire scheme, launched on 23 July, and 32,536 keys were issued, as some people ordered more than one key.
About 5,000 cycles are available across 330 locations in central London.
Cyclists have to pay an access fee from £1 a day plus usage time, with the first 30 minutes cycling free.
Cyclist Laurence, a city worker living in central London, said he had activated and set up an automatic £1 access top-up on all three keys he held.
He said: "I used my own key on Friday, Saturday and Sunday. I enjoyed using it, and I found it relatively easy to keep my journeys under 30 minutes, so I should have incurred no more than £3 for the three days.
"But I was in for a shock when I checked my online usage statement on Sunday. I had been charged £9 instead of £3. I contacted their customer service and it turns out that I was charged £1 per day times three days times all three keys."
He got TfL to unregister his two additional keys and they asked him to set up new accounts for each key with different email and card details.
Laurence added: "This is absolutely barking. I thought the whole point of ordering extra keys is so friends and family can use them, but not all at the same time."
On multiple keys, TfL's cycle-hire website says: "The new access fee will be charged against every key and not just the one that is used."
The TfL spokeswoman said: "We want to reassure members that anyone wishing to change their membership details, including whether they have additional keys linked to their account, should get in touch with our contact centre and we will make the necessary changes for them."