I have completed the dark brotherhood questline and had Cicero as my follower, then I downloaded the Dawngaurd DLC and completed that questline. Through the whole storyline I had Serana follow me as well as Cicero, but then even after the quest line she still follows me, so now I have Cicero and Serana. Why, is it a glich?
Many parts of the Dawnguard questline outright require Serana as your follower, and several of those quests are able to add her as your follower, even though you already have one.
The list of bugs associated with Serana is rather large. She's the single most complex NPC in the entire game, with an entire hidden quest (called
DLC1NPCMentalModel)acting under the hood to control her behavior, her dialog, etc., independent of the stuff that goes on as part of the actual quests.
It is incredibly likely that you have encountered a bug whereby the hidden quest got in an odd state, and didn't properly notice that she should have left after one of the forced-follower points. In that case, you may not even be able to get rid of her as a follower without resorting to console commands (and those only work if you're on the PC).