As resignation from a job is a voluntary act of the concerned employee, he/she has the right to withdraw it before it is accepted by the employer. Since you are in the notice period, your resignation is prospective. Therefore, the acceptance of it by the employer before the specified date in the resignation letter does not affect your right to withdraw it before that date. A resignation tendered to take effect from a future date will be in an inchoate state until the specified date expires. According to the ratio decidendi of the judgment of the Honorable High Court of Karnataka in Kushalnagar Works, Kudige v P. Nagaraju [1988 I LLN 1019], the refusal of the employer to permit the withdrawal of resignation is unjustified. Upon the withdrawal of resignation, the employee continues to be in service.
To avoid unnecessary conflict in this regard, send copies of your withdrawal to your immediate boss and the HR department as suggested by Ashutosh.