Double payment for single account

I advised ID mobile of my husbands death and asked for the account to be closed (which is held in my name).  There was £4.42 outstanding so I paid this.  Then I noticed a direct debit coming out for £7.50.  After further investigation it transpires that the plan was changed in July 2020 from £7.50 to £6.00 and we were advised that the direct debit would be altered accordingly instead double payments have been taken for the last 11 months.  I am now being told that I need to give 30 days notice and pay another £7.50.  So in total £75.