Drupal achieved this by requiring modules that get hosted on Drupal.org (the most popular site to find modules) be reviewed before they're published. Part of the review process requires verifying that the module (a) doesn't duplicate other modules and (b) doesn't belong as extra features added to another module. If your module doesn't work with the other modules on drupal.org, it will never be published. You're required to collaborate.
It works fairly well, although the review queue is long and you get people whining about how their module really deserves to be published.
I don't disagree with any of this, but would like to add that just because a module get published on drupal.org does not mean that (a) it works or (b) it's well-written.
The mainstream modules... the common ones used on thousands of sites, tend to be solid. There's a lot of half-baked crud once you get into the smaller or lesser-used ones.
This is true of any software library or framework that allows plugins, but it doesn't mean that the core idea itself is wrong.
For example, jQuery has dozens of gallery/slideshow and lightbox clones of widely varying quality, but jQuery itself is a brilliant foundation for Javascript development, including its plugin structure.
Indeed. I try to do rigorous reviews of modules (too much spare time at work), but I don't have enough time to install them and test every feature. I just check for obvious security holes and API abuses. And of course, once you've published one module you can make more without any review at all.
It works fairly well, although the review queue is long and you get people whining about how their module really deserves to be published.