When merging multiple config files it would be very useful to be able to replace an earlier definition with a later one - for example, a stable config with a definition for a stable package being overridden in a dev environment by a dev or beta version of the same package. The current merge implementation identifies the duplicates but then relies on Composer's Solver to find a compatible solution. I suggest a flag to change this with a 'replace' behaviour with 'last defined wins'.