user: Problem: NHS Business Services Authority (NHSBSA) issued 979,210 fines in 2016-17 - bringing in £13.3m - compared with 494,129 in 2015-16.
Patients can be fined a maximum of £100 for obtaining free medication after an exemption certificate has expired.
The British Medical Association (BMA) said those on low incomes and with long-term illnesses were often hit.
The Royal Pharmaceutical Society said fines often stemmed from patients not knowing or forgetting to renew their paperwork.
The NHSBSA said it was the patient's responsibility to ensure they were entitled to claim free-of-charge prescriptions.
The health body issued £13,277,877 in fines to people in England in 2016-17, up from £8,601,349 in 2015-16.
The data - obtained by the BBC under the Freedom of Information Act - has only been recorded since 2015 because responsibility for checks moved from local NHS trusts to the central NHSBSA in September 2014.
Patients should present an exemption certificate and sign the back of the prescription form to say that they are exempt of any charge.
The NHSBSA makes random checks when they think a claim for a free prescription has been made incorrectly.
If a patient cannot prove they were entitled to claim at the time, they are liable to pay a fine of the original prescription charge, plus five times that amount up to £100.
Brendan Brown, from the NHSBSA, said: "Where a patient makes a declaration on the back of the prescription form it's their responsibility to make sure that they do hold that exemption that they are declaring that they have.
"Fraudulent and other mistakes... do cost the NHS millions of pounds which could be spent on frontline NHS services."
England is the only country in the UK to still charge for prescriptions. The charge is currently £8.60 for each medicine or appliance dispensed.
The Royal Pharmaceutical Society said pharmacists largely rely on people's honesty when dispensing medicines free-of-charge.
Spokesman Neal Patel said: "What we've found is that people who've perhaps not done the correct paperwork, or their prescription exemption has run out of date because they've forgotten to renew it, those people are getting fined.
"We want to spend time making sure people can use their medicines, not policing the prescription charging system."
Louise Jarman, 45, from Chesterfield, has been diabetic since she was six years old and is entitled to free prescriptions, but was fined when she was told her exemption card had expired.
She said: "I received a letter out of the blue saying that I have not got an exemption card or a letter saying that I get free prescription, and they were fining me £50.
"I didn't know the card went out of date. They said they were fining me and I couldn't get any more prescriptions until I paid the fine.
"Eventually they dropped the charge but I was told I would have to pay for my previous prescription that I'd had, and any more until my card was renewed."
Dr Richard Vautrey, of the BMA, said no patient should have to pay for their care.
"Fining people should be a last resort. Often those people who may be fined are probably the least able to afford to pay.
"They're often people who have multiple conditions who may need regular prescriptions and that costs an awful lot of money."
Who gets free prescriptions?
Those over 60, under 16 or aged 16-18 and are in full-time education
Women who are pregnant or have had a baby in the last 12 months and have a valid maternity exemption certificate
People with a specified medical condition who hold a valid medical exemption certificate
Those with a continuing physical disability that prevents them from going out without help from another person and have a valid exemption certificate
Hold a valid war pension exemption certificate and the prescription is for your accepted disability
An NHS inpatient
You may also be entitled to free prescriptions if you or your partner receive certain allowances, such as income support
What was that article about?

A: Fines issued to patients in England claiming free prescriptions while not eligible have doubled, figures show.


The project, which will record every last detail of Orkney's St Magnus Cathedral, is to begin later this year.
Orkney Islands Council is currently looking for experts to carry out detailed scans of the internal and external structure of the cathedral.
A 3D record of the building can then be compared with future scans to detect any changes in its fabric.
It will allow for analyses of weathering, erosion, decay, moss build-up or shifts due to subsidence.
It is also hoped the scans can be used to aid research and enhance tourism by developing digital tours of the cathedral under separate proposals for a major exhibition on the history of Kirkwall.
The Society of the Friends of St Magnus Cathedral have given the project their backing, committing to contribute half of the costs of the laser scanning.
The society's secretary David Oddie said they were delighted to be contributing.
He said: "It goes right to the core of our chief aim - protecting and safeguarding the condition of this majestic building. The joint funding of this work is a reflection of the good relationship we've had with the council for many years."
The Romano-Gothic cathedral, built from red and yellow sandstone, is of international significance.
The foundations were placed in 1137, and the building, dedicated to Magnus Erlendsson, Earl of Orkney, took about 300 years to build.
It was assigned to the inhabitants of Kirkwall by King James III of Scotland in a charter dated 1486.

Sum: A 12th Century cathedral in Orkney is to undergo a laser scan as part of efforts to preserve the building.


Q: The men are Justinas Gubinas and Ovidijus Kavaliauska, who are both 22 of Rosehearty, Aberdeenshire, Remigijus Bidva, 24, and Nerijus Radavicius, 29, of Longhaven.
They have been charged with raping the woman at a farmhouse while she was drunk and incapable of giving consent.
They all deny the charge and have lodged a special defence of consent.
The men are alleged to have forced the woman, who cannot be named for legal reasons, to drink alcohol and induced her to carry out sex acts on 1 November 2014.
Giving evidence at the High Court in Aberdeen on Monday, the woman told the jury that the men took her to a house outside Fraserburgh in a car.
CCTV footage played to the court showed the woman chatting to a group of young men in a car park after leaving Deejays nightclub in Fraserburgh.
She described herself as being "beyond drunk" and claimed she was kicked in the back and slapped awake later that day.
The woman, who suffers from a borderline personality disorder, said video footage taken of sex acts made her feel "dirty".
The trial continues.

A: Four men have gone on trial accused of repeatedly raping a woman and filming sexual acts on their mobile phones.


Text: Roberts, 21, joined the Giants from Bradford Bulls in October 2014 and made his debut for the club in the opening game of the 2016 season.
"We have always been a club that has given chances to young British players who are hungry to succeed," head coach Paul Anderson told the club website.
"His challenge is to keep his place when our experienced guys return."
The Giants are second from bottom of the Super League, having failed to win any of their first three games.
They have also lost several key players to injury, including full-back Scott Grix and club captain Danny Brough, leaving Anderson reliant on inexperienced squad members.
"You can't always accept getting beat, but I think our attitude was very poor," Anderson told BBC Radio Leeds following Sunday's 36-18 defeat by Widnes Vikings.
"Consistency is something that we're always striving for, but that collective inexperience and collective poor attitude has cost us.
"We have so many people out in key positions that have been our strengths, and now our strengths have become our downfalls."
summary:

assistant: Huddersfield back-row forward Oliver Roberts has signed a new four-year deal with the Super League side.