async (ctx, next) => await next()

index.js

@@ -29,8 +29,12 @@ function compose(middleware){
    */
 
   return function (context, next) {
+    // last called middleware #
+    let index = -1
     return dispatch(0)
     function dispatch(i) {
+      if (i <= index) return Promise.reject(new Error('next() called multiple times'))
+      index = i
       const fn = middleware[i] || next
       if (!fn) return Promise.resolve()
       try {

test/test.js

@@ -174,9 +174,65 @@ describe('Koa Compose', function(){
 
     return compose(stack.map(co.wrap))({}).then(function () {
       throw 'promise was not rejected'
-    })
-    .catch(function (e) {
+    }).catch(function (e) {
       e.should.be.instanceof(Error)
     })
   })
+
+  // https://github.com/koajs/compose/pull/27#issuecomment-143109739
+  it('should compose w/ other compositions', function() {
+    var called = [];
+
+    return compose([
+        compose([
+            (ctx, next) => {
+                called.push(1)
+                return next()
+            },
+            (ctx, next) => {
+                called.push(2)
+                return next()
+            }
+        ]),
+        (ctx, next) => {
+            called.push(3)
+            return next()
+        }
+    ])({}).then(() => assert.deepEqual(called, [1, 2, 3]))
+  })
+
+  it('should throw if next() is called multiple times', function() {
+    return compose([
+      co.wrap(function* (ctx, next) {
+        yield next()
+        yield next()
+      })
+    ])({}).then(() => {
+      throw new Error('boom')
+    }, err => {
+      assert(/multiple times/.test(err.message))
+    })
+  })
+
+  it('should return a valid middleware', function () {
+    var val = 0
+    compose([
+      compose([
+        (ctx, next) => {
+          val++
+          return next()
+        },
+        (ctx, next) => {
+          val++
+          return next()
+        }
+      ]),
+      (ctx, next) => {
+        val++
+        return next()
+      }
+    ])({}).then(function () {
+      val.should.equal(3)
+    })
+  })
 })