Consider the example:

project(':api') {
  task foo << { println 'yo!' }
}

task main
main.dependsOn task(':api:foo')

Actual:
When running the 'main' task no exception is thrown even though we're adding the task that already exists.

Expected:
Build should fail as ':api:foo' is added twice. This way the user know that he misconfigured the dependsOn clause and can fix it by doing:

main.dependsOn ':api:foo'

You're not adding the same task twice. The parameter to task() is interpreted as a task name, not a task path. So the call to task(':api:foo') in the root project adds a task with name ':api:foo' to the root project, ie it's adding task '::api:foo', not ':api:foo'.

Instead, task() should either fail when a path is passed to it, or should treat its parameter as a path and add the task to the appropriate project. Either way, then you'd get an exception in the above case.

We should fix GRADLE-1631