Pension over payment

Dear Sir,

My father was a retired railway employee who got his pension after retirement in the year 2000 and died on 2003.

After that my mother informed the bank and the pension become family pension and my mother drew her family pension then on wards.

Suddenly on last month bank inform her verbally that bank made over payment to her for last several years and she have to deposit the excess amount as soon as possible other wise they will stop her pension.

Now.

1) Is bank able to stop the payment pf pension by informing her by a letter to recover the excess money?

2) My mother is unable to pay the amount at once rather she can repay by monthly basis can it be possible.

3) What action can bank taken to recover the excess payment?