Hello @Sab
Nice to hear from you again.
According to the website of BAMF a person holding Duldung is not permitted to work as self-employed as a matter of principle. Sorry for the bas news.
Source: http://www.bamf.de/EN/Infothek/FragenAntworten/ZugangArbeitFluechtlinge/zugang-arbeit-fluechtlinge-node.html
Best regards,
Thorgen