if you attempt to join a group when you're already in a group, you can't anymore.

public/language/en_GB/error.json

@@ -65,6 +65,7 @@
 	"group-name-too-short": "Group name too short",
 	"group-already-exists": "Group already exists",
 	"group-name-change-not-allowed": "Group name change not allowed",
+	"group-already-member": "You are already part of this group",
 
 	"post-already-deleted": "This post has already been deleted",
 	"post-already-restored": "This post has already been restored",

src/groups.js

@@ -615,15 +615,26 @@ var async = require('async'),
 	};
 
 	Groups.requestMembership = function(groupName, uid, callback) {
-		if (parseInt(uid, 10) > 0) {
-			db.setAdd('group:' + groupName + ':pending', uid, callback);
-			plugins.fireHook('action:group.requestMembership', {
-				groupName: groupName,
-				uid: uid
-			});
-		} else {
-			callback(new Error('[[error:not-logged-in]]'));
-		}
+		async.parallel({
+			exists: async.apply(Groups.isMember, uid, groupName),
+			isMember: async.apply(Groups.exists, groupName)
+		}, function(err, checks) {
+			if (!checks.exists) {
+				return callback(new Error('[[error:no-group]]'));
+			} else if (checks.isMember) {
+				return callback(new Error('[[error:group-already-member]]'));
+			}
+
+			if (parseInt(uid, 10) > 0) {
+				db.setAdd('group:' + groupName + ':pending', uid, callback);
+				plugins.fireHook('action:group.requestMembership', {
+					groupName: groupName,
+					uid: uid
+				});
+			} else {
+				callback(new Error('[[error:not-logged-in]]'));
+			}
+		});
 	};
 
 	Groups.acceptMembership = function(groupName, uid, callback) {