How to print only the unique lines in BASH?

How can I print only those lines that appear exactly once in a file? E.g., given this file:

mountain
forest
mountain
eagle

The output would be this, because the line mountain appears twice:

forest
eagle

• The lines can be sorted, if necessary.

Using awk:

awk '{!seen[$0]++};END{for(i in seen) if(seen[i]==1)print i}' file
eagle
forest