What version of an npm library is *actually* used?

I know that npm libraries, when installed, can install multiple versions of the same library in a hierarchical tree, like this:

a@0.1.0
  -> b@1.0
  -> c@2.0
       -> b@2.0

In the above, package a at version 0.1.0 is pulled in, and its dependencies b@1.0 and c@2.0. Similarly, c@2.0's dependency is pulled in, which is b@2.0.

I have heard from someone that, even though package b is installed at two different versions, only one of them is actually loaded into memory and used. I have also heard that this may or may not be the case for node deployments of javascript versus browser deployments.

So my question is: Is it true that only one package of b is loaded into memory? If it is, is this true for both node and browser, or are there differences?