I imagine in the medieval world it would be possible to walk past one's doppelganger and not even realize it, as one would not spend nearly the amount of time gazing upon one's own reflection as a modern person does.
How did the medieval world perceive reflections, and how did this change as mirrors became more common and cheaper to produce?
In the medieval world, reflections were more often seen in water and in shiny pots and pans. Mirrors, although hazy, existed among the rich, but most people used the more natural ways of seeing themselves, sometimes with fatal results. For example, a boy in medieval England drowned from gazing at his reflection in a barrel of water and fell in (Coroner's court rolls, city of London, 14th c.) Because these things existed for quite some time before the medieval period, it was nothing new, but medieval people did value appearance. A 13th c. book of manners cites that children should have clean nails, teeth and hands, as well as wearing fresh linen.
“Here endeth the mirror of perfection of a brother mirror. The Blessed Francis, wherein we may most sufficiently behold as in a glass the perfection of calling and profession.”
Leo of Assisi (died 1270) a Franciscan monk and confessor of Saint Francis of Assisi. Text from The mirror of perfection
Full text: The mirror of perfection
This backs /u/14thCenturyHood in the assessment that almost any reflective surface was used.
hi! here are a few related posts that may be of interest...
Historiography of body image beyond the wide-spread availability of mirrors?
When did mirrors become readily available? How did people see themselves before they were around?