Will it be considered duplicate submission in a venue, if we knew that the article will be rejected for sure in another venue?
Formally, yes.  Until you have actually received the accept/reject decision, your paper is still under submission and cannot be submitted elsewhere.

On the other hand, you can avoid this conflict (at least in principle) by formally withdrawing your original submission. Normally I would advise against withdrawing so late, since you'd arguably be wasting the PC's time.  But if the work has already been done and the outcome is clear, the PC is just wasting your time by making you wait two weeks for the official decision.

If you do withdraw and submit elsewhere before results of the first conference are announced, you should make the situation very clear to the new PC chair. In particular, you should forward the formal acknowledgment of your withdrawal.  There is a nontrivial chance that the two PCs overlap, and that someone will flag the "dual submission" without realizing that it was withdrawn. 

Finally: If the reviews are so obviously negative, you might consider giving yourself more than 15 days to improve the paper before submitting again.