Gita - Chapter 1, Verse 3

 paśyaitāṁ pāṇḍu-putrānām
āchārya mahatīṁ chamūm
vyūḍhāṁ drupada-putreṇa
tava śhiṣhyeṇa dhīmatā 

Meaning :

The mighty army of the sons of Pandus have been assembled for battle, along with your trained pupil, Dhrstadyumna, son of Drupada, Master.

Comment :

Duryodhana is showing the army of Pandavas to Dronacharya and saying that they have assembled on the battlefield with the support of Dhrstadyumna, son of Drupada, who was also trained under Dronacharya. By doing this, Duryodhana is seen to be in a dilemma as Dhrstadyumna was born with the intention to kill Dronacharya.

There is a backstory to the relation between Dronacharya & Dhrstadyumna, which is discussed below.

Drupada and Drona were friends and once Drupada offers half of his kingdom to Drona when Drupada gets the throne. However, when Drona due to his bad financial situation later on asks help to Drupada, who became the King by virtue of his father's death,  he refuses and teases Drona. 

Drona gets insulted & as time passed by, Drona is employed to teach the Kauravas & Pandavas. As a price for the teachings, he asks them to defeat Drupada, in which they succeed and Drona keeps half of Drupada's kingdom as promised.

Now, Drupada gets humiliated & to take vengeance, performs a yajna to beget a son capable enough to kill Drona. As a result, the twins were born Dhrstadyumna & Draupadi. 

Knowing all this, Dronacharya takes up the duty to train Dhrstadyumna too. Draupadi later gets married to Arjuna & so Dhrstadyumna is fighting for the Pandavas. He is also the Supreme Commander in Chief of Pandavas.

Keeping all these incidents in mind, Duryodhana loses his confidence and asks Dronacharya not to show any mercy.