Duplication in open source
One thing that people constantly seem to carp on about in Open Source circles is the frequency with which there are several apps that do the same thing. They then generally go on to say that the developers should work on whatever their favourite application is. The implicit assumption here is that having multiple apps is wasteful, but I don't agree that this is the case.
One of the well known tenets of computer science (first set down by Fred Brooks in 'the mythical man month' ) is that adding more developers to a project will not always make things go faster. There are a number of reasons for this including the length of time it takes people to get up to speed with what they're doing, but also the difficulty of coordinating the effort.
In open source development, work is generally distributed between developers who are scattered around the world. Communication is generally by email or IRC and only rarely are there opportunities like aKademy where people can get together and talk face to face. The limited communication bandwidth between developers is also reduced by the fact that the most productive developers tend to code rather than talk! The net effect of these factors is that the difficulties outlined by Fred Brooks are even more difficult to overcome in open source development than in traditional 'cathederal' style development.
A further factor exists in open source, and that is the ability of one project to reuse code from another freely. This is very common though it often takes the form of reusing ideas rather than directly copying code. Since the ideas and concepts behind all apps are visible to everyone because we have the source, developments in one application can help the progress of another. If application 'A' comes up with a cool way of solving a problem then there's a good chance you'll see application 'B' using a variant of it soon after. Taking all these things together, you can see that having two apps for a problem is not necessarily a waste.
A final thought to consider is that most open source developers are volunteers. They are writing code because they like to write code, open source projects are often simply a way for like-minded people to get together and write cool stuff. If one day someone thinks I wonder how you'd write something that does XYZ then an application that does XYZ is what will result. But the motivation wasn't to produce the application, so much as to see how you would do it.