early outs for privs

no need to check if empty array is passed in, happens if there are no
unread topics
remove dupe cids before checking for privileges

src/privileges/categories.js

@@ -68,6 +68,14 @@ module.exports = function(privileges) {
 	};
 
 	privileges.categories.filter = function(privilege, cids, uid, callback) {
+		if (!cids.length) {
+			return callback(null, []);
+		}
+
+		cids = cids.filter(function(cid, index, array) {
+			return array.indexOf(cid) === index;
+		});
+
 		async.parallel({
 			allowedTo: function(next) {
 				helpers.allowedTo(privilege, uid, cids, next);

src/privileges/posts.js

@@ -77,6 +77,9 @@ module.exports = function(privileges) {
 	};
 
 	privileges.posts.filter = function(privilege, pids, uid, callback) {
+		if (!pids.length) {
+			return callback(null, []);
+		}
 		posts.getCidsByPids(pids, function(err, cids) {
 			if (err) {
 				return callback(err);

src/privileges/topics.js

@@ -76,6 +76,10 @@ module.exports = function(privileges) {
 	};
 
 	privileges.topics.filter = function(privilege, tids, uid, callback) {
+		if (!tids.length) {
+			return callback(null, []);
+		}
+
 		var keys = tids.map(function(tid) {
 			return 'topic:' + tid;
 		});