Can you put little bird attachment on a character that already has attainted attachment on that character?

Yes. Effects that give or remove an icon (or any other characteristic) act like effects that increase or decrease strength: they are cumulative. So Little Bird and Attainted would cancel each other out, regardless of the order in which they were attached. The duration of the effects is not relevant: if Selyse's ability (granting an intrigue icon) is used on a character with Attainted, the net result is that both effects cancel each other out (as long as both are active).

Thanks

You're welcome.