Now before you go blaring "Yes! She is! What are you on?!" hear me out~
I have a theory-supported by several points-that may prove otherwise. Besides the obvious and simple fact they look and act nothing alike, mind.
First of all-when Kikyou is called back to the living world, Kagome's body spews out what appears to be *hundreds* of souls...rather then just one.
When Kagome's will pulls them back-Kikyou is still walking and "Kikyou" so to speak, if revengelike. If Kagome's soul had been Kikyou's, one or the other would not be walking. Instead both walk...and Kikyou's soul just can't stabilize in a clay person container-so it winds up she needs help.
When Kikyou passes away for the final time, not one soul returns to Kagome when they all break loose. Not one.
Kaede mentions a little later that Kikyou probably had made an unconscious wish on the Shikon no Tama as she burned away with it; to see Inuyasha again. And since Kagome was 'fated' to meet the hanyou 50 years later in her era the thing granted her wish by causing her soul just hitch a ride in Kagome via the tama, because the tama reappeared in Kagome.
Other small details-Kikyou called out in Kagome's mind to *not call my name*, giving the impression she was seperate from Kagome already.
Kanna's mirror was unable to hold all of Kagome's "soul" because there was obviously more than one and thus filled the mirror to the point of cracking.
The theory after all of this is: Kagome is a multi-soul 'reincarnation', she was not just one person's, but many people's. Very likely because the tama itself held many, many souls within it, and needed many souls to keep it in limbo. And Kikyou's was a forced part of that due to the tama.
SO, that's my theory...tell me, do you think Kagome is really Kikyou's reincarnation?