There are typically two situations where two symbols may refer to the same object.
R objects usually have copy-on-write semantics. This is an optimisation that ensures that objects are only copied if needed. When you copy a vector, no memory is actually copied until you modify either the original object or the copy is modified.
Note that the copy-on-write optimisation is an implementation detail that is not guaranteed by the specification of the R language.
Assigning an uncopyable object (like an environment) creates a reference. These objects are never copied even if you modify one of the references.
Examples
# Reassigning an uncopyable object such as an environment creates a
# reference:
env <- env()
ref <- env
is_reference(ref, env)
#> [1] TRUE
# Due to copy-on-write optimisation, a copied vector can
# temporarily reference the original vector:
vec <- 1:10
copy <- vec
is_reference(copy, vec)
#> [1] TRUE
# Once you modify on of them, the copy is triggered in the
# background and the objects cease to reference each other:
vec[[1]] <- 100
is_reference(copy, vec)
#> [1] FALSE