Charm person and look-alikes / identical twins

[Mrs Camelot]

Hi,

If a caster casts Charm person on someone (we'll call him Bob), then an identical copy (be it through illusion, body-switching or flesh-sculpting), of the caster turns up and speaks to Bob, will Bob treat anything the copy says favourably or will he have some mystical feeling the spell is no longer working/the copy is a fake? Assuming Bob has no way of knowing the copy is not the original caster.

Thanks.

[The Morphling]

The rules say nothing on this.

I would rule on a case-by-case basis as it came up in my game.