Originally Posted by Evil Iggy
The Doctor states that it doesn't need saying. When she asks the Duplicate he leans into her and whispers into her ear.
Again, the natural assumption is that he was going to say "I love you" but why the whispering then? She immediately kisses him passionately and he returns the kiss so it wouldn't really be a secret.
If you think back to the episode Forest of the Dead, when Professor River Song whispers into his ear it is later revealed that she was whispering his name. Is it possible that the Duplicate Doctor was whispering because he was telling Rose his true name?
But why would Rose kiss him passionately if all he'd done was whisper his true name? It's never worked for me...