No, the error is in IF clause. Here's the procedure code:

IF @IncludeHosts = 0
BEGIN
SELECT
 UO.UserID,
 U.UserName,
 U.DisplayName,
 U.FirstName,
 U.LastName,
 U.FirstName + ' ' + U.LastName AS FullName
FROM
 dbo.UsersOnline UO INNER JOIN dbo.Users U ON UO.UserID = U.UserID INNER JOIN dbo.UserPortals UP ON U.UserID = UP.UserID
WHERE
 UO.PortalID = @PortalID AND UO.UserID = U.UserID AND UP.Authorised = 1 AND U.IsSuperUser = 0 -- Inner Join takes care of SU = 0, but for sanity.
END
ELSE
BEGIN
SELECT DISTINCT
 UO.UserID,
 U.UserName,
 U.DisplayName,
 U.FirstName,
 U.LastName,
 U.FirstName + ' ' + U.LastName AS FullName
FROM
 dbo.UsersOnline UO INNER JOIN dbo.Users U ON UO.UserID = U.UserID, dbo.UserPortals UP
WHERE
 UO.PortalID = @PortalID AND UO.UserID = U.UserID AND UP.Authorised = 1

I have solved this problem by replacing 'SELECT' in the first case with 'SELECT DISTINCT'.