In The Notebook, Allie appears to be standing by the glass doorway looking at herself in the room. What is happening here?
Could someone explain this please, as I don't quite understand it.
...Allie appears to be standing by the glass doorway looking at herself in the room.....Then near the end she stood watching her previous self.
That is definitely Allie, but not looking at herself through the glass door. She is looking into some other room.
Those two shots are different scenes at different times : first one is our present Allie who is sitting on a wheelchair,second one is also our present Allie, who after sometime maybe wanted to look what's happening in the adjoining room.
As you said, Allie was suffering from Alzheimer's disease and dementia, she could not keep track of people, everything in general.
It is hard to say what is going on in one's mind, specially for people with dementia,so that part of the movie is really ambiguous in the sense what she was thinking at that time.
First she was made to sit on the chair,there she had a blank expression. Maybe she was phasing in and out, wondering why she was there. Maybe she remembered/recognised Noah and was wondering if he was in the other room and hence peeked through the glass door or maybe not.