diff --git a/include/mapnik/datasource_cache.hpp b/include/mapnik/datasource_cache.hpp
index 74a70c992..9827886f9 100644
--- a/include/mapnik/datasource_cache.hpp
+++ b/include/mapnik/datasource_cache.hpp
@@ -34,6 +34,7 @@
 
 // stl
 #include <map>
+#include <set>
 
 namespace mapnik {
 
@@ -55,7 +56,7 @@ private:
     ~datasource_cache();
     std::map<std::string,boost::shared_ptr<PluginInfo> > plugins_;
     bool registered_;
-    std::vector<std::string> plugin_directories_;
+    std::set<std::string> plugin_directories_;
 };
 }
 
diff --git a/src/datasource_cache.cpp b/src/datasource_cache.cpp
index 1e16c33d4..d1ec3d630 100644
--- a/src/datasource_cache.cpp
+++ b/src/datasource_cache.cpp
@@ -166,8 +166,7 @@ void datasource_cache::register_datasources(std::string const& str)
 #ifdef MAPNIK_THREADSAFE
     mutex::scoped_lock lock(mutex_);
 #endif
-    // TODO - only push unique paths
-    plugin_directories_.push_back(str);
+    plugin_directories_.insert(str);
     if (mapnik::util::exists(str) && mapnik::util::is_directory(str))
     {
         boost::filesystem::directory_iterator end_itr;
@@ -195,9 +194,15 @@ void datasource_cache::register_datasources(std::string const& str)
 
 bool datasource_cache::register_datasource(std::string const& filename)
 {
-    bool success = false;
     try
     {
+        if (!mapnik::util::exists(filename))
+        {
+            MAPNIK_LOG_ERROR(datasource_cache)
+                    << "Cannot load '"
+                    << filename << "' (plugin does not exist)";
+            return false;
+        }
         boost::shared_ptr<PluginInfo> plugin = boost::make_shared<PluginInfo>(filename,"datasource_name");
         if (plugin->valid())
         {
@@ -213,7 +218,7 @@ bool datasource_cache::register_datasource(std::string const& filename)
                 MAPNIK_LOG_DEBUG(datasource_cache)
                         << "datasource_cache: Registered="
                         << plugin->name();
-                success = true;
+                return true;
             }
         }
         else
@@ -229,7 +234,7 @@ bool datasource_cache::register_datasource(std::string const& filename)
                 << "Exception caught while loading plugin library: "
                 << filename << " (" << ex.what() << ")";
     }
-    return success;
+    return false;
 }
 
 }