Duch, who is 69, was handed a commuted sentence of 19 years at the end of his atrocity crimes trial in 2010.
Defense attorneys for three jailed regime leaders have challenged the authenticity of documents from the well-established organization.
Nil Nonn, head judge of the Trial Chamber, stopped him and reminded him to keep his questions limited to the case at hand—No. 002.
Bou Meng and Chhum Mey spend less time at the trial of Khmer Rouge leaders these days, and more time at the torture center they both survived.
The UN sent its expert for the court to Cambodia this week to meet with government and court officials over a UN-appointed judge.
Overcoming such trauma is not easy and going back into the past does not help either, Sam Keo said.
Bou Meng and Chhum Mey spend less time at the trial of Khmer Rouge leaders these days, and more time at S-21 museum.
UN Secretary-General Ban Ki-moon nominated Kasper-Ansermet to replace a German judge who resigned last year.
The Documentation Center has collected nearly 1 million documents on the Khmer Rouge since 1995.
Kasper-Ansermet was to have filled a vacancy created by the departure of German judge Siegfried Blunt - who resigned late last year.
A UN spokesman told VOA Khmer this week the government has an obligation to fill vacant posts at the court.
The UN appointed a reserve judge to the position, Laurent Kasper-Ansermet, but that appointment has not been confirmed.