South African Chad le Clos says he considers himself the winner of the 2016 Olympic 200m freestyle even though he officially holds only the silver medal.
The ban was given after Sun, a three-time Olympic champion, failed to allow a doping test, in September 2018, which was considered a second offence after he had served a three-month ban for taking a prohibited substance in 2014.
"I know when I look at myself in the mirror that I on gold in that freestyle in Rio in 2016 as I know I've done my best," Le Clos told BBC Sport Africa.
The 29-year-old, who will be competing at this year's Olympics, believes that all doping offences should carry a life ban.
"I know I'm a champion in the mirror because I've done everything I could possible and have respected the rules," he added.
"I do want that title. If I break my leg tomorrow or slip down the stairs and never swim again, I'd like them to say 'two gold medals, two silvers', not 'three silvers, one gold at the Olympics'."
Le Clos' sole gold came after beating the great Michael Phelps of the United States in the 200m butterfly at the 2012 Olympics.
Even though Sun was unaffected by any doping issues at the time of the 2016 race, Le Clos believes the Chinese should be stripped of his gold medal.
"I don't know him - he could be a good guy - but if you've cheated and taken any drugs, you should be banned for life," said Le Clos, who is also an ambassador for the World Anti-Doping Agency.
"That's always been my stance and it's nothing personal towards anybody else - it's just we need to make swimming a clean sport."
"If I won the gold in Rio, my life would be different and I would be too. It would be a different legacy. I've lost that moment and I've lost the financial benefits that come with an Olympic champion."
Sun has previously said that his three-month ban in 2014 came after he took the prohibited stimulant Trimetazidine as part of a prescribed drug for heart palpitations which he had suffered since 2008.
He says he was unaware that it was added to the list of banned substances shortly prior to his failed test.
Regarding the missed test of 2018, Sun told an appeal hearing in November 2019 that he did not submit to a doping test as the testers failed to prove their identity when they arrived at his home.
He also denied a vial containing his blood samples, which had initially been collected in September 2018, was smashed with a hammer.
Sport's highest legal body Cas found that Sun had 'acted recklessly … when he refused to allow the blood samples to leave with the Sample Collection Personnel, causing the abortion of the out-of-competition anti-doping control of 4-5 September 2018'.
This week, Sun's lawyer took to Chinese social media to defend his client: "I can only say forever - Sun Yang did not violate the rules and there were no violations detected in the results [from doping tests]."
Le Clos, meanwhile, will be hoping he can secure a second gold in Tokyo.
"I've put in the work and I have a lot of support around the world and in South Africa - and that's going to give me the extra 5-10% I need at the Olympics to hopefully bring maximum medals," he said.
"I'm not going to make any predictions I don't know what the results, going to be."