Joël Boissard, who works for Swiss broadcaster RTS, was fined, ordered to pay court costs and given a further suspended fine after being found guilty in early November, according to news agencies.
The incident occurred last year when Boissard, who had recently moved house, received two sets of voting documents for federal and cantonal elections on March 8th 2015.
Assuming the online system would prevent him from voting twice, he tried to do so – and succeeded.
Boissard immediately contacted the electoral authorities to report what he had done and ask them to explain the anomaly, he told news agencies.
Three weeks later, he discovered the authorities had reported him to Geneva's public prosecutor for electoral fraud.
In making its judgement, the court in Bern decided Boissard had intentionally acted illegally, and did not take into account his job as a journalist, said ATS.
However Boissard argued that he acted in good faith, wanting to simply draw attention to a matter of public interest.
On Tuesday Reporters without Borders (RSF), which works to protect press freedoms around the world, condemned the court's decision, saying in a statement that it was “an extraordinary attack on the media's freedom and duty to inform the public”.
“According to his lawyer, Jamil Soussi, the European Court of Human Rights has in the past ruled in favour of journalists who broke the law solely because it was the only way to draw attention to serious problems,” it added.
“In this case, Boissard used the only way possible to confirm that the electronic voting system was flawed.”
Boissard and RTS intend to appeal to Switzerland's highest criminal court “on the grounds that his conviction constitutes a grave violation of media freedom”, said RSF.
The Federal Criminal Court will hear the case in January.