Probably your supply is down and she latches on, realizes 'no milk' latches off and then is asleep and forgets so she latches on again. You could try seeing if she'll wake up enough to drink some water. Or, might she be teething? My kids all nurse like that when they are teething. (Ow.)
I do want DD to wean - I tandem nursed my two DSs for about 3 months and it did not work for me (or them, since it wasn't working for me). I started offering her a 'special hug' instead of nursing for times during the day when she is upset and it went well for a couple of days and then yesterday she got really upset about it and I let her nurse. Sigh. I guess I will talk with her about it today and maybe try again in a few weeks. She is 33 months.