The city of Chicago said Thursday it has dismissed some 23,000 outstanding duplicate vehicle sticker tickets and will refund motorists who have already paid for an additional 12,000 duplicates dating to the early 1990s.

The decision was made Wednesday, city officials said, five months after a ProPublica Illinois and WBEZ investigation revealed that, on nearly 20,000 occasions since 2007, motorists had been been cited more than once on the same day, in apparent violation of the municipal code.

The city’s decision affects even more drivers because its analysis of ticket data extends to duplicate citations issued since 1992, adding 15 years’ worth of sticker tickets to the equation.

Motorists in black and low-income neighborhoods see the highest rate of ticketing for vehicle sticker violations, including duplicates, ProPublica Illinois and WBEZ have found.

In one case, Chicago police officers issued three $200 sticker tickets to a motorist who had parked her vehicle outside her apartment at a gated, low-income housing development, all within 78 minutes.

The city is notifying motorists who paid for duplicate tickets so they can claim refunds online or by mail and is working to upgrade its technology to automatically void new duplicate tickets, Kristen Cabanban, a Finance Department spokeswoman, said in an emailed statement.

Officials did not announce the decision until Thursday afternoon, the same day a lawsuit was filed alleging the city had been issuing duplicate sticker tickets just to generate revenue from fines and late penalties.

“To be clear, we’ve been working on this for months,” Cabanban said. “We first had to understand the [scope] of the issue and then work to come up with a solution, followed by working with our IT vendor to perform the programming. Yesterday was the culmination of those efforts.”

The lawyer who filed the lawsuit, Jacie Zolna, said he didn’t “believe it for a minute.”

“This is them finding out at 4 p.m. last night I was going to sue them, and now they’re scrambling around to make it seem like they are doing it out of the goodness of their heart. … It just proves that these types of lawsuits are the only thing that will make the city change these illegal practices,” he said.

Zolna, who filed the lawsuit in Cook County Circuit Court, has sued the city before over ticket issues. Last year, he won a nearly $39 million settlement on behalf of motorists who did not receive adequate notice for automatic red-light and speed-camera tickets in the city. In another case, filed in July, Zolna accused the city of violating a state cap on ticket penalties. That case is pending.

Vehicle sticker requirements are fairly unique among large U.S. cities. Chicago requires motorists to buy annual stickers — typically between $88 and $139, depending on vehicle weight — and affix them to the windshields of their cars. Revenue from sticker sales goes to fund street repair.

At $200, the sticker ticket is one of the most expensive in the city and has contributed to hundreds of millions of dollars of debt and thousands of bankruptcies, particularly in black neighborhoods. With late penalties, one ticket can cost $488.

Lauren Nolan, research director at Woodstock Institute, a nonprofit policy organization that issued a report on ticketing disparities over the summer, called the city’s decision to dismiss the duplicate tickets a sign of progress but said more work lies ahead to make the system less punishing to the poor.

“This represents a drop in the bucket as it relates to the ticket issues more broadly,” she said, “but it’s good residents are no longer being hit with purely administrative errors which should never have happened in the first place.”

It’s unclear if the city’s decision to dismiss unpaid tickets and issue refunds will render the most recent lawsuit moot. The lawsuit also took aim at duplicate tickets for another type of vehicle compliance violation that vexes Chicago motorists every year: expired license plates. An analysis of city ticket data by ProPublica Illinois and WBEZ show close to 100,000 instances of duplicate citations for expired license plates since 2007.

One of the two plaintiffs in the city sticker lawsuit is a Chicago firefighter, Rodney Shelton, who filed for bankruptcy in 2015 after he lost his driver’s license and faced losing his job over unpaid tickets — including some duplicates. Shelton had racked up thousands of dollars in ticket debt on a broken-down car he had left on a vacant lot next to a relative’s home in the Garfield Park neighborhood.

“It was a no-brainer to say enough is enough,” he said. “I’ve already gone through enough. It has to get better.”

The city’s move came as momentum grows for reform of the city’s ticketing and debt collection practices. The Chicago City Council this month approved a plan to allow motorists to buy city stickers in four-month installments. The measure, from City Clerk Anna Valencia, is meant to help low-income drivers afford stickers so they won’t get ticketed.

The City Council also approved a Law Department proposal to forgive some debt and create more affordable payment plans for motorists who file for bankruptcy under Chapter 7. Most drivers who file for bankruptcy over unpaid tickets use Chapter 13, which typically ends without any debt relief.

Next month, aldermen may tackle yet another set of proposed reforms that would limit license suspensions over unpaid tickets and allow community service instead of debt repayment.