Outstanding tax demand by CPC for the second time!

Hello,

I am writing this on behalf of my father who retired from a PSU bank. He has been filing IT returns on time and pays tax in advance (even before filing return) and same was done for AY 2016-17. But he received outstanding tax demad notice via mail in August 2016 to which he left a response on incometaxefiling with all details like challan and BSR etc. and now in February 2017 he received another mail with oustanding tax demand! I would like to request an opinion on process that needs to be followed.

Thanks in advance.

Vishal