URGENT: She has a new UK visa; is her old one still valid?

[Boobaaa]

Hello,
Mum applied for a two year visa 3 weeks ago as she wanted to come visit me here in February and stay for a few months. When she applied, she still had a valid UK visa expiring in March 2020. Now she's been issued with a visa starting in January 2020 and her old visa has NOT been cancelled. Now she wants to visit the UK in December instead, can anyone confirm that her old visa is certainly valid? Is it possible to have two valid visas at the same time???? When she applied, she was expecting to be issued with a visa starting from November and her old one to be cancelled. Now we are really confused!
I'd really appreciate someone who is familiar with these matters to respond ASAP as I don't want her to encounter any problems once at the airport. Thank you.