Version: (using KDE KDE 3.5.4) Installed from: Debian testing/unstable Packages Wether or not KDM should allow multiple logins per user is discussed in 103046. Whatever will be the decision on that, I think the typical use is one session per user and KDM should reflect that: instead of storing only one previous user and preselecting it, it should maintain a longer list of recent users and choosing the most active user for preselection, but _excluding_ the currently logged in users. My wife and I use the same workstation and only 50% of the time the preselection is good, I thing the method above would improve it.
sounds a bit like overkill, but in principle you're right.
KDM is unmaintained and not used in KDE Plasma 5. SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/