You can use this sed command to do it:
$ sed 's/\([A-Za-z]\)\1\+/\1/g' file.txt
Example
Using your above sample input I created a file, sample.txt.
$ sed 's/\([A-Za-z]\)\1\+/\1/g' sample.txt
NAME
nice - run a program with modified scheduling priority
SYNOPSIS
nice [-n adjustment] [-adjustment] [--adjustment=adjustment] [comand [a$