diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -427,6 +427,8 @@
     'ArcanistRepositoryQuery' => 'repository/query/ArcanistRepositoryQuery.php',
     'ArcanistRepositoryRef' => 'ref/ArcanistRepositoryRef.php',
     'ArcanistRepositoryRemoteQuery' => 'repository/remote/ArcanistRepositoryRemoteQuery.php',
+    'ArcanistRepositoryURINormalizer' => 'repository/remote/ArcanistRepositoryURINormalizer.php',
+    'ArcanistRepositoryURINormalizerTestCase' => 'repository/remote/__tests__/ArcanistRepositoryURINormalizerTestCase.php',
     'ArcanistReusedAsIteratorXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistReusedAsIteratorXHPASTLinterRule.php',
     'ArcanistReusedAsIteratorXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistReusedAsIteratorXHPASTLinterRuleTestCase.php',
     'ArcanistReusedIteratorReferenceXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistReusedIteratorReferenceXHPASTLinterRule.php',
@@ -1455,6 +1457,8 @@
     'ArcanistRepositoryQuery' => 'Phobject',
     'ArcanistRepositoryRef' => 'ArcanistRef',
     'ArcanistRepositoryRemoteQuery' => 'ArcanistRepositoryQuery',
+    'ArcanistRepositoryURINormalizer' => 'Phobject',
+    'ArcanistRepositoryURINormalizerTestCase' => 'PhutilTestCase',
     'ArcanistReusedAsIteratorXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistReusedAsIteratorXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistReusedIteratorReferenceXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
diff --git a/src/repository/remote/ArcanistRepositoryURINormalizer.php b/src/repository/remote/ArcanistRepositoryURINormalizer.php
new file mode 100644
--- /dev/null
+++ b/src/repository/remote/ArcanistRepositoryURINormalizer.php
@@ -0,0 +1,159 @@
+<?php
+
+/**
+ * Normalize repository URIs. For example, these URIs are generally equivalent
+ * and all point at the same repository:
+ *
+ *   ssh://user@host/repo
+ *   ssh://user@host/repo/
+ *   ssh://user@host:22/repo
+ *   user@host:/repo
+ *   ssh://user@host/repo.git
+ *
+ * This class can be used to normalize URIs like this, in order to detect
+ * alternate spellings of the same repository URI. In particular, the
+ * @{method:getNormalizedPath} method will return:
+ *
+ *   repo
+ *
+ * ...for all of these URIs. Generally, usage looks like this:
+ *
+ *   $norm_a = new ArcanistRepositoryURINormalizer($type, $uri_a);
+ *   $norm_b = new ArcanistRepositoryURINormalizer($type, $uri_b);
+ *
+ *   if ($norm_a->getNormalizedPath() === $norm_b->getNormalizedPath()) {
+ *     // URIs appear to point at the same repository.
+ *   } else {
+ *     // URIs are very unlikely to be the same repository.
+ *   }
+ *
+ * Because a repository can be hosted at arbitrarily many arbitrary URIs, there
+ * is no way to completely prevent false negatives by only examining URIs
+ * (that is, repositories with totally different URIs could really be the same).
+ * However, normalization is relatively aggressive and false negatives should
+ * be rare: if normalization says two URIs are different repositories, they
+ * probably are.
+ *
+ * @task normal Normalizing URIs
+ */
+final class ArcanistRepositoryURINormalizer
+  extends Phobject {
+
+  const TYPE_GIT = 'git';
+  const TYPE_SVN = 'svn';
+  const TYPE_MERCURIAL = 'hg';
+
+  private $type;
+  private $uri;
+  private $domainMap = array();
+
+  public function __construct($type, $uri) {
+    switch ($type) {
+      case self::TYPE_GIT:
+      case self::TYPE_SVN:
+      case self::TYPE_MERCURIAL:
+        break;
+      default:
+        throw new Exception(pht('Unknown URI type "%s"!', $type));
+    }
+
+    $this->type = $type;
+    $this->uri = $uri;
+  }
+
+  public static function getAllURITypes() {
+    return array(
+      self::TYPE_GIT,
+      self::TYPE_SVN,
+      self::TYPE_MERCURIAL,
+    );
+  }
+
+  public function setDomainMap(array $domain_map) {
+    foreach ($domain_map as $key => $domain) {
+      $domain_map[$key] = phutil_utf8_strtolower($domain);
+    }
+
+    $this->domainMap = $domain_map;
+    return $this;
+  }
+
+
+/* -(  Normalizing URIs  )--------------------------------------------------- */
+
+
+  /**
+   * @task normal
+   */
+  public function getPath() {
+    switch ($this->type) {
+      case self::TYPE_GIT:
+        $uri = new PhutilURI($this->uri);
+        return $uri->getPath();
+      case self::TYPE_SVN:
+      case self::TYPE_MERCURIAL:
+        $uri = new PhutilURI($this->uri);
+        if ($uri->getProtocol()) {
+          return $uri->getPath();
+        }
+
+        return $this->uri;
+    }
+  }
+
+  public function getNormalizedURI() {
+    return $this->getNormalizedDomain().'/'.$this->getNormalizedPath();
+  }
+
+
+  /**
+   * @task normal
+   */
+  public function getNormalizedPath() {
+    $path = $this->getPath();
+    $path = trim($path, '/');
+
+    switch ($this->type) {
+      case self::TYPE_GIT:
+        $path = preg_replace('/\.git$/', '', $path);
+        break;
+      case self::TYPE_SVN:
+      case self::TYPE_MERCURIAL:
+        break;
+    }
+
+    // If this is a Phabricator URI, strip it down to the callsign. We mutably
+    // allow you to clone repositories as "/diffusion/X/anything.git", for
+    // example.
+
+    $matches = null;
+    if (preg_match('@^(diffusion/(?:[A-Z]+|\d+))@', $path, $matches)) {
+      $path = $matches[1];
+    }
+
+    return $path;
+  }
+
+  public function getNormalizedDomain() {
+    $domain = null;
+
+    $uri = new PhutilURI($this->uri);
+    $domain = $uri->getDomain();
+
+    if (!strlen($domain)) {
+      return '<void>';
+    }
+
+    $domain = phutil_utf8_strtolower($domain);
+
+    foreach ($this->domainMap as $domain_key => $domain_value) {
+      if ($domain === $domain_value) {
+        $domain = $domain_key;
+        break;
+      }
+    }
+
+    return $domain;
+  }
+
+}
diff --git a/src/repository/remote/__tests__/ArcanistRepositoryURINormalizerTestCase.php b/src/repository/remote/__tests__/ArcanistRepositoryURINormalizerTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/repository/remote/__tests__/ArcanistRepositoryURINormalizerTestCase.php
@@ -0,0 +1,84 @@
+<?php
+
+final class ArcanistRepositoryURINormalizerTestCase
+  extends PhutilTestCase {
+
+  public function testGitURINormalizer() {
+    $cases = array(
+      'ssh://user@domain.com/path.git' => 'path',
+      'https://user@domain.com/path.git' => 'path',
+      'git@domain.com:path.git' => 'path',
+      'ssh://user@gitserv002.com/path.git' => 'path',
+      'ssh://htaft@domain.com/path.git' => 'path',
+      'ssh://user@domain.com/bananas.git' => 'bananas',
+      'git@domain.com:bananas.git' => 'bananas',
+      'user@domain.com:path/repo' => 'path/repo',
+      'user@domain.com:path/repo/' => 'path/repo',
+      'file:///path/to/local/repo.git' => 'path/to/local/repo',
+      '/path/to/local/repo.git' => 'path/to/local/repo',
+      'ssh://something.com/diffusion/X/anything.git' => 'diffusion/X',
+      'ssh://something.com/diffusion/X/' => 'diffusion/X',
+    );
+
+    $type_git = ArcanistRepositoryURINormalizer::TYPE_GIT;
+
+    foreach ($cases as $input => $expect) {
+      $normal = new ArcanistRepositoryURINormalizer($type_git, $input);
+      $this->assertEqual(
+        $expect,
+        $normal->getNormalizedPath(),
+        pht('Normalized Git path for "%s".', $input));
+    }
+  }
+
+  public function testDomainURINormalizer() {
+    $base_domain = 'base.phabricator.example.com';
+    $ssh_domain = 'ssh.phabricator.example.com';
+
+    $domain_map = array(
+      '<base-uri>' => $base_domain,
+      '<ssh-host>' => $ssh_domain,
+    );
+
+    $cases = array(
+      '/' => '<void>',
+      '/path/to/local/repo.git' => '<void>',
+      'ssh://user@domain.com/path.git' => 'domain.com',
+      'ssh://user@DOMAIN.COM/path.git' => 'domain.com',
+      'http://'.$base_domain.'/diffusion/X/' => '<base-uri>',
+      'ssh://'.$ssh_domain.'/diffusion/X/' => '<ssh-host>',
+      'git@'.$ssh_domain.':bananas.git' => '<ssh-host>',
+    );
+
+    $type_git = ArcanistRepositoryURINormalizer::TYPE_GIT;
+
+    foreach ($cases as $input => $expect) {
+      $normalizer = new ArcanistRepositoryURINormalizer($type_git, $input);
+
+      $normalizer->setDomainMap($domain_map);
+
+      $this->assertEqual(
+        $expect,
+        $normalizer->getNormalizedDomain(),
+        pht('Normalized domain for "%s".', $input));
+    }
+  }
+
+  public function testSVNURINormalizer() {
+    $cases = array(
+      'file:///path/to/repo' => 'path/to/repo',
+      'file:///path/to/repo/' => 'path/to/repo',
+    );
+
+    $type_svn = ArcanistRepositoryURINormalizer::TYPE_SVN;
+
+    foreach ($cases as $input => $expect) {
+      $normal = new ArcanistRepositoryURINormalizer($type_svn, $input);
+      $this->assertEqual(
+        $expect,
+        $normal->getNormalizedPath(),
+        pht('Normalized SVN path for "%s".', $input));
+    }
+  }
+
+}