Replacing Duplicates With Underlines In Strings

Problem description & analysis
Below are a set of strings:
Here here here we go!
 
So they're finally here, performing for you
If you know the words, you can join in too
Put your hands together, if you want to clap
As we take you through this monkey rap!
We are trying to replace all duplicates of each word with underlines, as shown below:
Here ____ ____ we go!
 
So they're finally ____, performing for you
If ___ know the words, ___ can join in too
Put your hands together, __ ___ want to clap
As __ take ___ through this monkey rap!
Solution
Write the following script p1.dfx in esProc:

Explanat...