How do package.el handles "duplicate packages"?

[Eduardo Ochs <eduardoochs@gmail.com>]

Hi list,

where can I find information on how package.el handles packages that
are listed twice in the buffer that `M-x list-packages' creates? The
file package.el is very big and I find its code hard to follow, so any
hints like "read the commments in the places foo and bar" would help a
lot...

Here is an example (to make this question more interesting)...
"afternoon-theme" is one of these packages, and if I define these two
functions

  (defun my-p1 () (interactive) (setq my-p1 (tabulated-list-get-id)))
  (defun my-p2 () (interactive) (setq my-p2 (tabulated-list-get-id)))

and I run `M-x my-p1' on the line with the first "afternoon-theme" and
`M-x my-p2' on the second, then I can inspect the variables my-p1 and
my-p2 with:

  (describe-variable 'my-p1)
  (describe-variable 'my-p2)

One of them has these entries,

  :version (0 1)
  :archive "nongnu"

and the other has these:

  :version (20140104 1859)
  :archive "melpa"

and if I run this,

  (setq my-ps
    (cl-loop for (name pdesc) in package-archive-contents
             if (eq name 'afternoon-theme)
             collect (list name pdesc)))

  (describe-variable 'my-ps)

I see only one entry in package-archive-contents - the one in MELPA...

  Thanks in advance,
    Eduardo Ochs
    http://angg.twu.net/#eev
    http://angg.twu.net/eepitch.html