Landlord kept part of the deposit

[B K]

I ended the tenancy agreement with my landlady by mutual agreement. I moved out of the flat on the 2nd April however the agreement ends on the 16th April. The landlady has found another tenant who moved in on the 5th April. When returning my deposit the landlady deducted the rent from the 6th to the 16th April and transferred the rest back to me. My question is whether she is allowed to charge 2 people for 1 room? Is there anything I can do to get the money back?

Thanks.