add another duplicated-handler cfail test

tests/cfail/duplicated-handler-2.rs

+#![deny(warnings)]
+#![feature(proc_macro)]
+#![no_std]
+
+#[macro_use(task)]
+extern crate cortex_m_rtfm as rtfm;
+extern crate stm32f103xx;
+
+use rtfm::{app, Threshold};
+
+app! {
+    device: stm32f103xx,
+
+    resources: {
+        static ON: bool = false;
+    },
+
+    tasks: {
+        EXTI0: {
+            enabled: true,
+            path: exti0,
+            priority: 1,
+            resources: [ON],
+        },
+    },
+}
+
+fn init(_p: init::Peripherals, _r: init::Resources) {}
+
+fn idle() -> ! {
+    loop {}
+}
+
+fn exti0(_r: EXTI0::Resources) {}
+
+// ERROR can't override the task handler specified in `app!`
+task!(EXTI0, exti1);
+//~^ error cannot find value `EXTI0`
+
+fn exti1(_t: &mut Threshold, _r: EXTI0::Resources) {}