Groups pushing for the impeachment of Brazil's President Dilma Rousseff are using a large rubber duck as a mascot.
But the design is similar to one by Dutch artist Florentijn Hofman, who told the BBC the Brazilian replica constitutes copyright infringement.
The owner of the factory that produced both ducks denied any wrongdoing.
Versions of the same giant inflatable rubber duck designed by Mr Hofman have travelled the world since 2007, calling in Japan, New Zealand and Brazil, among many other countries.
The version that has appeared in protests in Brazil closely resembles Mr Hofman's, although it has crosses for eyes.
It also has the slogan "Chega de pagar o pato" across its chest, a Portuguese expression meaning "We won't pay for the duck any more" or "We won't pay for what is not our fault any more".
The giant duck was commissioned by a powerful Brazilian industrial group, FIESP, to use in protests against corruption and high taxes from last September.
But it has made a number of appearances in demonstrations against the president in recent months.
"It is exactly our design and our specific technical patterns," Mr Hofman told BBC Brasil. "Changing the eyes doesn't change our technical design of the shape and beak."
Before it appeared as part of an exhibition in Brazil, a version of Mr Hofman's duck was produced in a Sao Paulo factory.
The owner of the factory, Denilson Sousa, told the BBC they also produced the new duck, but said the design was not copied.
"I would not put our reputation at risk," he said. "We have experience in this kind of job and this is a very simple design. Why wouldn't we spend four hours redesigning it?"
But Mr Hofman said the factory "made a very unwise decision" and that he considered it "illegal use of the exact design and therefore copyright infringement".
A FIESP spokesman said they had been reassured the design was original.
On Tuesday, the group released 5,000 rubber ducks near the main national congress building in the capital, Brasilia, and took out full-page adverts in national newspapers using an image of a duck.
Opposition lawmakers want to remove Ms Rousseff over claims she manipulated government accounts to hide a growing deficit.
Her party's coalition partners quit government on Tuesday, a step which may ensure she lacks enough support to stave off impeachment.

The man, in his late 40s, was involved in a housing dispute with bailiffs at the property on Stapleton Hall Road in Stroud Green.
Police officers were called to the scene at midday after receiving reports the man was armed with a weapon.
Shortly after they arrived the explosion took place, leaving him with severe burns. Three police officers sustained minor injuries.
The basement and parts of the ground floor and shed in the back garden were on fire for several hours. No neighbouring houses were damaged.
The cause of the explosion is not yet known and there have been no arrests. The Met said it was not looking for anyone else in connection with the incident.
Witness Paul Pixton tweeted: "Something massive is happening on my street, some kind of altercation on street followed by loud explosion."
An artist famous for a giant rubber duck that pops up around the world says a version of his work used by Brazilian protesters amounts to plagiarism.
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A man has been critically injured in an explosion at a house in north London.