One of my players got the (fun) idea that her character could be able to smell the difference between a doppelganger copy and the real person.
She's a shifter, so in theory she has an excellent sense of smell. (Or so I've been ruling. I don't know if that's RAW).
Are there any written rules that say what a doppelganger smells like? (And if that's any different than the copied person?) While we're playing D&D 5E, content from other editions would be welcome.
Best Answer
Fun idea
It is a fun idea. I don't think it would work.
I have an even more fun idea: don't tell the player it won't work - let her think it can and have lots of fun when she fingers the innocent.
Rules
Interestingly, "polymorph" is not defined anywhere in the rules. It only gets its definition from the local context "... into a Small or Medium humanoid ..." and "... each form ..." and normal English usage "having, assuming, or passing through many or various forms, stages, or the like." The fact that there are spells that use the word does not inform the usage here except in a general way - the doppelganger is not using a polymorph spell; it is using the Shapechanger ability.
"Appearance" means several things, the most relevant being "the state, condition, manner, or style in which a person or object appears; outward look or aspect" - there is no indication that this is limited to visual appearance only: the doppelganger changes its visual, aural, tactile, and chemical appearance (i.e. what it smells and tastes like).
So here is what it doesn't do: by inference, it gets everything else.
You don't get much closer to someone than this - presumably they smell good enough to mate with.
Rationale
The doppelganger is a species that has evolved to blend in with any small or medium humanoid species to thrive and reproduce. It can do this so effectively that it can engage in sexual activity without the other party suspecting.
Some of these humanoid species have great senses of smell (e.g. orcs, goblins) some of them keep dogs or other domestic animals with great senses of smell.
If a doppelganger doesn't smell like a member of the species its impersonating; it will be found out.
Acquired Smells
GMJoe wrote in a comment:
Since comments don't last and this is a really good point I have added it in.