I'm sure those are one and the same. She should clarify that with them. If all of them were registered, letters from them would have all their names on.
If it's just addressed to her, they think she lives there alone, and if they think she lives alone, she will probably be liable for the whole amount, because she will currently be getting a discount for the fact that they think she lives alone. Whoever lives there, is liable for council tax. It's the law to pay it, you can't have 1 person liable for everybodies council tax I don't think.
I told them myold flat mate moved out 12 months ago which he did, I was just lazy in updating them and they refunded me for all the months I'd overpaid, because I'm not liable for his council tax, only my own.