Using sed, how can I remove duplicate letters from HEADERS within a text file?

NNAAMMEE
       nice - run a program with modified scheduling priority

SSYYNNOOPPSSIISS
       nice     [-n    adjustment]    [-adjustment]    [--adjustment=adjustment] [command [a$

Above is a an example. I want the output after parsing with sed to be:

NAME
       nice - run a program with modified scheduling priority

SYNOPSIS
       nice     [-n    adjustment]    [-adjustment]    [--adjustment=adjustment] [command [a$