gistfile1.txt

task :a do
  puts "hi"
end

Rake.application.lookup(:a).clear

task :a do
  puts "hello"
end

###################################

namespace :n1 do
  task :a do
    puts "hi"
  end
end

Rake.application.lookup('n1:a').clear

namespace :n1 do
  task :a do
    puts "hello"
  end
end