Add filter that uniquifies collections.

diff --git a/src/main/java/net/pterodactylus/sone/template/UniqueElementFilter.java b/src/main/java/net/pterodactylus/sone/template/UniqueElementFilter.java
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+++ b/src/main/java/net/pterodactylus/sone/template/UniqueElementFilter.java
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+/*
+ * Sone - UniqueElementFilter.java - Copyright © 2011 David Roden
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package net.pterodactylus.sone.template;
+
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import net.pterodactylus.util.template.Filter;
+import net.pterodactylus.util.template.TemplateContext;
+
+/**
+ * Filter that reduces a collection to a {@link Set}, removing duplicates.
+ *
+ * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+ */
+public class UniqueElementFilter implements Filter {
+
+       /**
+        * {@inheritDoc}
+        */
+       @Override
+       public Object format(TemplateContext templateContext, Object data, Map<String, String> parameters) {
+               if (!(data instanceof Collection<?>)) {
+                       return data;
+               }
+               return new HashSet<Object>((Collection<?>) data);
+       }
+
+}