Members of the congregation saying “Nashhadu (we bear witness) and “Haqqan (indeed)” when the imam praises Allaah

The Sunnah is that when the imam recites Du’aa’ al-Qunoot
during the prayer, the members of the congregation should say Ameen to his
du’aa’. But when the imam is praising Allaah, it is better for them to
remain silent, because the imam’s praise is sufficient. As for some of the
members of the congregation saying, Nashhadu (we bear witness) or “Ya
Allaah” and so on, there is no basis for this. 

Shaykh Bakr Abu Zayd said: 

Some mistakes and
innovations are sometimes done during the du’aa’ and dhikr, such as the
following: …

 45. Answering the dhikr during the qunoot of the imam by
saying “haqqan (indeed) or “sadaqta (you have spoken the truth)” or “sadaqan
wa ‘adalan (that is right and true)” or “ashhadu (I bear witness)” or “haqq
(truth)” and so on. There is no basis for saying any of these things. 

Tasheeh al-Du’aa’, p. 419-423.