The brace means he is the first player in Bundesliga history to score seven goals in his first three appearances in the competition.
Incredibly, each of the forward's first seven shots in Germany's top flight have ended as goals.
The 19-year-old netted a hat-trick on his debut against Augsburg on January 18 after being introduced as a 56th-minute substitute, following his move from Salzburg earlier in the month.
He followed that up with two more goals in the thumping 5-1 win over Cologne on January 24 after coming on with 25 minutes to go.
He was included from the start against Union Berlin and repaid coach Lucien Favre's faith, scoring in either half to help Dortmund move up to third in the table, three points behind leaders Bayern Munich.