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Method #1

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$

Method #2

There is also this method which will remove all the duplicate characters:

$ sed 's/\(.\)\1/\1/g' file.txt 

Example

$ sed 's/\(.\)\1/\1/g' sample.txt 
NAME
    nice - run a program with modified scheduling priority

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