The Replacement.

[Dyoll]

In The Replacement Xander gets split in to two people so he has a twin. Nicholas Brendon acutely has a twin I found that quite interesting dose any one no anything about that episode is was it just a coincidences ?

[The Kinslayer]

No it´s no coincidence. They decided to take advantage of the fact that Nick has a twin, Kelly. So that´s at least one reason why the episode was done. Don´t know if they´d skip the whole idé if he hadn´t had a twin though. But it´s not like Kelly plays one of the Xanders the whole show. He´s just in it then the two Xanders are together (or something like that).

It´s fun then peole are twins and use it like this, then you´re not expecting it. But it can be confusing. It took me quite a while to get that there are two Ashmores (Sean and Aaron). At first I thought that it was the same guy playing Iceman in X-men and Jimmy Olsen on Smallville. To make it even more confusing Sean has appeared on two Smallville episodes as Eric Summers.

[Blondie Bear]

I'd also heard somewhere that Nick was sick while they were shooting some episode or another so Kelly stood in for him. I don't remember which episode it was, but I'm pretty sure I heard that in a commentary.

Edit:

I found it on IMDB. Kelly Donovan stood in for Nick Brendan in "Intervention." (Now I'm going to have to watch that episode again!) If I remember right, though, it wasn't for the speaking parts, just for the standing around in the Magic Box parts.