Bug#562398: anna: fails if multiple versions of a udeb in Packages file

[Frans Pop <elendil@planet.nl>]

Package: anna
Severity: serious

Yesterday I uploaded some packages, including base-installer and hw-detect. 
Today during a test install from a local build this resulted in:

                     Failed to load installer component
       loading base-installer failed for unknown reasons. Aborting.

The syslog does not show much info, but this is interesting:
[...]
anna: DEBUG: retrieving base-installer 1.104
anna: DEBUG: retrieving base-installer 1.104
anna: DEBUG: retrieving bootstrap-base 1.104
[...]
anna: DEBUG: retrieving disk-detect 1.74
anna: DEBUG: retrieving disk-detect 1.74
[...]

So the error is probably the result of anna trying to install the same 
package twice.

I ran anna a second time and that finished without errors and the rest of 
the installation also completed without problems, so the impact is 
limited.

The packages file on my local mirror shows the cause of the problem. It 
contains two versions of both base-installer (1.103, 1.104) and 
disk-detect (1.73, 1.74). Both are arch:all packages, so this is probably 
the simple result of the recent changes in the archive and the packages 
not yet being built for all arches.

We need to at least make sure anna does not try to process the same package 
twice. It already seems to return the correct (highest) version.

The actual fix is probably in libd-i rather than anna itself.

This issue will only manifest for packages that have both arch:any and 
arch:all udebs, and only while it has not been built for all arches.
So the problem will be fairly rare [1], but IMO it should still be fixed as 
it looks extremely fatal.