Changing one file multiple times, executes it multiple times each run

[leoc]

First change, rake task is run once
Second change of the same file, rake task is run twice (interleaved)
Third change of the same file, rake task is run thrice (interleaved as well)

Should it not be run once per change?

Is there a way to run a task exclusively. So to wait and run it, when the first invoked process finished?