Subdirectories are added more than once

When git lb-checkout for a certain package is called more than once (on the same or a different branch), it is added several times to list of packages to compile, causing cmake to fail.

It can be reproduced with:

lb-dev --nightly lhcb-master Moore/master
cd MooreDev_master/
git lb-use Rec
git lb-checkout Rec/master Tr/PatPV
git lb-checkout Rec/master Tr/PatPV
make