Fix slots definitions in Gauss

@dpopov, this fixes the issue we observed with the rebuilds of lhcb-gauss-dev (and others).

In principle we should not be able to add a project twice to the list, but somehow the check was fooled in this case. Then a minor difference in the the way the configuration of a slot gets parsed for the regular nightly builds and when you do a restart made the restart actually count (correctly) the data packages twice, resulting in a checkout error the second time the package was checked out.