Near the end of “Multiplication”, we see Duusu appear in front of Felix, confused, after all, Duusu was not aware of what was happening when Gabriel renounced him back in “Strike Back”, all he saw was two people talking, so it makes sense he was confused by seeing another person wear his miraculous. But the thing that seems to be confusing people is Duusu suddenly saying Nathalie’s name to Felix, after all, aren't kwamis supposed to not say their past holders names?

Now, from what we have seen so far, all the kamis have the spell which prevent them from talking about their holders identities to others, but there is an exception. If the person in front of them already knows the identity of their past holder then the kwamis are fully capable of talking about them, but despite not knowing their true identities the kwamis can still talk about their holders through another name, like how Tikki is still able to call Marinette Ladybug in front of others

Duusu was not the only kwami who we saw talk about their past holder, Niiri the eagle kwami in the New york special, talked about his past holder, “Gilbert” ( Gilbert du Motier, Marquis de Lafayette ). The reason he said his past holders name to Gabriel was because of the same situation that Duusu was in, Niiri likely still had the same spell on him to prevent him from talking about his past holder, but Gabriel already knew who the past holder of the eagle miraculous was thanks to his research through the history files

So yeah, given Felix pieced together the fact that Gabriel was Shadowmoth, it would not have taken much for him to realize that Nathalie was Mayura. Duusu told Felix Nathalie's name because he could, because Felix already knew Mayura’s identity.