Van der Merwe's rugby career began in George, South Africa, playing for Outeniqua High School before gaining provincial recognition in youth tournaments.

Transitioning to the professional level, he joined the Blue Bulls' academy in 2014 and made his mark with the South Africa under-20 squad at the World Rugby U20 Championship.

[1] Duhan was born to Dulene (mother) and Alhan (father) van der Merwe, in George, a coastal town in the Western Cape province of South Africa.

He attended and played rugby for Outeniqa High School (Hoërskool Outeniqua), earning several provincial colours by representing the SWD Eagles at youth tournaments.

In 2011, he represented SWD [South Western Districts] at the Under-16 Grant Khomo Week held in Queenstown, appearing in all three matches and scoring a try in their 25–8 victory over KwaZulu-Natal.

He was named in the South Africa Schools team at the conclusion of the tournament,[5] and made two starts for them in the Under-18 International Series, helping them to victories over France[6] and England.

[30] In their final match of the regular season against bottom team UCT Ikey Tigers, Van der Merwe scored six of his side's eleven tries in a 100–25 victory.

[40] On 4 January 2021, it was announced that Van der Merwe would move to England to sign for Worcester Warriors ahead of the 2021–22 Premiership Rugby season on a long-term deal.

[42] Only hours after having his contract terminated at Worcester Warriors it was confirmed that Van der Merwe would return to Edinburgh on a long-term deal.

Van der Merwe was a late call-up to the South Africa Under-20 squad that participated at the 2014 IRB Junior World Championship in New Zealand,[46] replacing Rohan Janse van Rensburg who picked up an injury in South Africa's 61–5 victory against Scotland in the opening round in Pool C.[47] Van der Merwe was an unused replacement for their 33–24 victory over hosts New Zealand in their second match,[48] but started their final pool match, a 21–8 victory over Samoa[49] to help the team finish top of their pool to progress to the semi-finals.

He didn't feature in their 32–25 win over New Zealand at that stage,[50] but was named on the bench for the final against England and came on as a replacement in the second half, with South Africa losing 20–21[51] to finish as runners-up in the competition.

[57] This was followed by another brace in the final match of the championship in Paris, the second coming in 84th minute of play to secure a dramatic last-gasp win, Scotland's first in France since 1999.

Van der Merwe was selected in Scotland's 33-player squad for the 2023 Rugby World Cup in France;[60] however, they exited the tournament in the pool stage, failing to beat both Ireland and South Africa.

[63] After performing well in the tour's warm up games, scoring five tries in four appearances, he was selected in the starting line up for the first Test and played 70 minutes as the Lions won 17-22.