I put Axell into play :

Response: After Ser Axell Florent enters play, choose and discard 1 duplicate from play

My opponent has the Red Viper in play already with a duplicate attached :

Renown. Immune to events and character abilities.

Can i still discard a duplicate as the response is directly targetting Red Viper but instead targetting ANY duplicate ?

Daenarys said:

Yes, you can. Duplicates have no text and hence have no immunities.

And duplicates, like attachments, power and gold tokens, or abilities are not covered by immunity (FAQ 3.20 Scope of Immunity).