Guest,
To the extent that GTD duplicates your firm or office's existing system, that duplication isn't necessarily bad. I was doing a lot of that duplication before, because I want a separate set of reminders about what I am responsible for, rather than what the entire firm or the team on any single case is responsible for. As examples, my firm uses docketing software, which generates calendars of litigation and other deadlines. I've always (even in my pre-GTD days) taken deadlines that affect me or that I am responsible for and put them on my calendar. And even in my pre-GTD days, whenever I felt like I didn't have a mental handle on everything that I was responsible for, I'd take a few minutes and write out a list of them: motion on x case re x; discovery on y case to y; respond to z's letter; etc. It's just that now I feel like I have a deliberate system in place to capture and track all of these things on a constant basis, not just when things get hectic.
Perhaps the single greatest benefit I enjoy from GTD is knowing now what I'm responsible for and what I want to accomplish, and knowing that I have a single point of reference where all of that information is. Nothing is lost, at least in theory, if I'm vigilant about it. I no longer wake up at 2 am because I just remembered that told someone 2 weeks ago I'd have something for them tomorrow, and that is priceless.
This system isn't mutually exclusive to whatever calendar, conflicts-check, case-management or contacts system you have. In fact, you might be able to adapt those existing systems to a GTD-focused practice. The mindset is just as important as the method used to make it happen.