Bug 22003

Description of problem:
Just had a weird failure of the installer earlier today on a system where I had to resize a partition to make place for a new partition. That new partition was thus inserted between existing partitions, which (as the installer notified me) led to a renumbering of partitions. But then the installer gave a weird error about the partition numbers (unfortunately didn't have a camera and was in a hurry then) and re-started the partitioning tool a second time on top of the already running instance of it, so it was visible twice on top of each other. 

I suspect that it was due to the fact that the installer tried to use the mount points that were defined and probably remembered by partition number without taking into account the renumbering it had made itself.

And just in case you'd wonder how I even came to find myself in that situation: that's because I had left the old mga5 system on its partition to install mga6 in an lvm and therefore needed a /boot partition. I had happily defined my mount points and thought I could use a pre-existing vfat data partition of freedos for /boot but then the installer told me that I couldn't use it and needed a native linux filesystem for /boot (i guess for making symlinks) so I had to resize that partition smaller and make a new partition for /boot which led to the renumbering. 

Steps to Reproduce (guessed):
1. define mount points in the partition management tool of the installer
2. resize a partition smaller and insert a new partition in the space freed by that operation, at a place on the disk that will change the partition numbers of  some higher numbered partition that had already a mount point defined
3. failure and installer weirdly starts a second instance of the partitioning tool on top of the old one