Closes #27 by adding a test for repeat builds

Author: webketje

README.md

@@ -98,7 +98,7 @@ There are 2 ways to create collections & they can be used together:
         },
         pattern: 'news/**/*.html',
         sortBy: 'pubdate',
-        reverse: true,
+        reverse: true
       },
       services: 'services/**/*.html'
     })

test/index.js

@@ -388,4 +388,26 @@ describe('@metalsmith/collections', function () {
         done()
       })
   })
+
+  it('should not add duplicates on repeat builds', function (done) {
+    const metalsmith = Metalsmith('test/fixtures/noconfig').use(collections())
+
+    Promise.all([
+      new Promise((resolve, reject) => {
+        metalsmith.process((err) => {
+          if (err) reject(err)
+          resolve(metalsmith.metadata().collections)
+        })
+      }),
+      new Promise((resolve, reject) => {
+        metalsmith.process((err) => {
+          if (err) reject(err)
+          resolve(metalsmith.metadata().collections)
+        })
+      })
+    ]).then((results) => {
+      assert.deepStrictEqual(results[0], results[1])
+      done()
+    })
+  })
 })