Duffek was international representative of the Austrian SPÖ and director of the Renner Institute. He died on Sunday.
He had worked closely with the pan-European Party of European Socialists for the past two decades.
Reaction to his death was swift, with the PES President Sergei Stanishev saying: "I am deeply saddened by the death of Karl, a great friend and a great colleague.
"He had the DNA of a social democrat, he was passionate about the history of our movement and always keen to contribute to the debates about the challenges we face today, particularly on growing populism and inequalities and on inequalities in education."
Speaking on Tuesday, the former Bulgarian Prime Minister added, "He intensely believed in the power of education to empower the poor. We have learned a lot from him."
The latest common project Duffek worked on with the PES was the fourth European Training Academy in Vienna in June.
Stanishev said: "Karl has died too young. We have lost the skills and the commitment of an incredible politician.
"But, most of all, we have lost a friendly colleague, someone with a good sense of humour. A friend who will be dearly missed and we'll always remember. We wish his beloved ones the strength to overcome this loss."
Duffek has been acclaimed by others as a "great analytical thinker" who worked on numerous publications on Europe, including the rise of populism.
A PES source said, "He worked tirelessly to bring the Karl-Renner-Institute forward and help build and develop FEPS. He was of the opinion that education was the key to empowerment; and all who are capable to study should have the opportunity, regardless of the fact if one is rich or poor.
"He especially dedicated himself to this."
He added, "Karl had an impressive historic knowledge of social democracy and could bring any debate back to its fundaments and to the core values of social democracy.
"Always reasoning and with calm. But most of all he was a friendly colleague, with a good sense of humour. A friend who will be dearly missed and we will always remember."
Further comment came from the SOLIDAR organisation, which said it "mourns" Duffek who, it said, had been a "longstanding friend, comrade and supporter of SOLIDAR."
Born in 1962 in Vienna, Duffek studied political and social sciences and became active in the Socialist Students Union of Austria. He later joined the Karl-Renner-Institute in 1998 and was appointed its director a year later.
As International Secretary of the Austrian Social Democratic Party and Vice-President and Treasurer of Foundation for European Progressive Studies - FEPS he worked closely with SOLIDAR.