Incorrect package duplicate error when using/missing .git extension
## Incorrect package duplicate error when using/missing .git extension

### Issue description
dbt seems to treat packages as not identical (but with duplicate name) if one reference has the `.git` file extension and another does not. This is problematic when adding a dependency that uses a package a project already includes.

### Results
I had a project that was pulling the `dbt-utils` package. When I set up the `dbt-event-logging` package, despite pulling the same version/release, dbt returned the following error:
```
Found duplicate project dbt_utils. 
This occurs when a dependency has the same project name as some other dependency.
```
My `packages.yml` looked like:
```
packages:
  - git: "https://github.com/fishtown-analytics/dbt-utils"
    revision: 0.1.12

  - git: "https://github.com/fishtown-analytics/dbt-event-logging.git"
    revision: 0.1.5
```
The `package.yml` file in dbt-event-logging looks like:
```
packages:
  - git: "https://github.com/fishtown-analytics/dbt-utils.git"
    revision: 0.1.12
```
The problem was resolved by adding the `.git` suffix to the dbt-utils package in my project. 

### System information
The output of `dbt --version`:
```
installed version: 0.10.1
   latest version: 0.11.1
```

The operating system you're running on: Windows

The python version you're using: 3.6.0


diff --git a/core/dbt/task/deps.py b/core/dbt/task/deps.py
index 3e282c25d..aaf37979e 100644
--- a/core/dbt/task/deps.py
+++ b/core/dbt/task/deps.py
@@ -219,6 +219,13 @@ class GitPackage(Package):
         self._checkout_name = hashlib.md5(six.b(self.git)).hexdigest()
         self.version = self._contents.get('revision')
 
+    @property
+    def other_name(self):
+        if self.git.endswith('.git'):
+            return self.git[:-4]
+        else:
+            return self.git + '.git'
+
     @property
     def name(self):
         return self.git
@@ -368,14 +375,38 @@ def _parse_package(dict_):
 
 
 class PackageListing(AttrDict):
+    def __contains__(self, package):
+        if isinstance(package, basestring):
+            return super(PackageListing, self).__contains__(package)
+        elif isinstance(package, GitPackage):
+            return package.name in self or package.other_name in self
+        else:
+            return package.name in self
+
+    def __setitem__(self, key, value):
+        if isinstance(key, basestring):
+            super(PackageListing, self).__setitem__(key, value)
+        elif isinstance(key, GitPackage) and key.other_name in self:
+            self[key.other_name] = value
+        else:
+            self[key.name] = value
+
+    def __getitem__(self, key):
+        if isinstance(key, basestring):
+            return super(PackageListing, self).__getitem__(key)
+        elif isinstance(key, GitPackage) and key.other_name in self:
+            return self[key.other_name]
+        else:
+            return self[key.name]
 
     def incorporate(self, package):
         if not isinstance(package, Package):
             package = _parse_package(package)
-        if package.name not in self:
-            self[package.name] = package
+
+        if package in self:
+            self[package] = self[package].incorporate(package)
         else:
-            self[package.name] = self[package.name].incorporate(package)
+            self[package] = package
 
     @classmethod
     def create(cls, parsed_yaml):
@@ -488,16 +519,16 @@ class DepsTask(ProjectOnlyTask):
 
         while pending_deps:
             sub_deps = PackageListing.create([])
-            for name, package in pending_deps.items():
+            for package in pending_deps.values():
                 final_deps.incorporate(package)
-                final_deps[name].resolve_version()
-                target_config = final_deps[name].fetch_metadata(self.config)
+                final_deps[package].resolve_version()
+                target_config = final_deps[package].fetch_metadata(self.config)
                 sub_deps.incorporate_from_yaml(target_config.packages)
             pending_deps = sub_deps
 
         self._check_for_duplicate_project_names(final_deps)
 
-        for _, package in final_deps.items():
+        for package in final_deps.values():
             logger.info('Installing %s', package)
             package.install(self.config)
             logger.info('  Installed from %s\\n', package.nice_version_name())
diff --git a/test/integration/006_simple_dependency_test/test_simple_dependency.py b/test/integration/006_simple_dependency_test/test_simple_dependency.py
index 1eac417ca..cd0ac9847 100644
--- a/test/integration/006_simple_dependency_test/test_simple_dependency.py
+++ b/test/integration/006_simple_dependency_test/test_simple_dependency.py
@@ -1,5 +1,7 @@
+import os
 from test.integration.base import DBTIntegrationTest, use_profile
 
+
 class TestSimpleDependency(DBTIntegrationTest):
 
     def setUp(self):
@@ -63,6 +65,67 @@ class TestSimpleDependency(DBTIntegrationTest):
         self.assertEqual(created_models['view_model'], 'view')
         self.assertEqual(created_models['view_summary'], 'view')
 
+
+class TestSimpleDependencyWithDuplicates(DBTIntegrationTest):
+    @property
+    def schema(self):
+        return "simple_dependency_006"
+
+    @property
+    def models(self):
+        return "test/integration/006_simple_dependency_test/models"
+
+    @property
+    def packages_config(self):
+        # dbt should convert these into a single dependency internally
+        return {
+            "packages": [
+                {
+                    'git': 'https://github.com/fishtown-analytics/dbt-integration-project'
+                },
+                {
+                    'git': 'https://github.com/fishtown-analytics/dbt-integration-project.git'
+                }
+            ]
+        }
+
+    @use_profile('postgres')
+    def test_postgres_simple_dependency_deps(self):
+        self.run_dbt(["deps"])
+
+
+class TestRekeyedDependencyWithSubduplicates(DBTIntegrationTest):
+    @property
+    def schema(self):
+        return "simple_dependency_006"
+
+    @property
+    def models(self):
+        return "test/integration/006_simple_dependency_test/models"
+
+    @property
+    def packages_config(self):
+        # dbt-event-logging@0.1.5 requires dbt-utils.git@0.1.12, which the
+        # package config handling should detect
+        return {
+            'packages': [
+                {
+                    'git': 'https://github.com/fishtown-analytics/dbt-utils',
+                    'revision': '0.1.12',
+                },
+                {
+                    'git': 'https://github.com/fishtown-analytics/dbt-event-logging.git',
+                    'revision': '0.1.5',
+                }
+            ]
+        }
+
+    @use_profile('postgres')
+    def test_postgres_simple_dependency_deps(self):
+        self.run_dbt(["deps"])
+        self.assertEqual(len(os.listdir('dbt_modules')), 2)
+
+
 class TestSimpleDependencyBranch(DBTIntegrationTest):
 
     def setUp(self):
['core/dbt/task/deps.py', 'test/integration/006_simple_dependency_test/test_simple_dependency.py']