Good use of the theme! I like the unexpected use of the mirror as a trap, and the doppleganger's appearance.

I'd recommend breaking up the large paragraphs into smaller ones, to make them easier to read. It also gives you better control over the pacing of the story.

Aside from that, the story scanned well and the ending was ominous!

Very nice. I liked the ending, and you got in a good amount of characterisation given how little space there was. To be hyper-pedantic, there were a few minor grammatical errors, but nothing that affected comprehension or the flow of the story.