How to display unique nomenclature parameters?

Not a solution with the nomencl package but an alternative using the acro package (as per request in a comment to the question): acro allows the creation of “local” list of acronyms which only list the acronyms that were used in between two uses of acbarrier. In addition a list of all acronyms can be created. This way there is no need to define acronyms mutiple times.

printacronyms % complete list
acbarrier
...
printacronyms[local] % local list
acbarrier
...
printacronyms[local] % another local list
acbarrier

These barriers can be called automatically, for example by adding them to the section command. The example below demonstrates this using preto (provided by the etoolbox package).

documentclass{article}

usepackage{acro}
acsetup{
  barriers/use = true ,
  barriers/reset = true
}

newcommandprintsectionacronyms[1]{%
  printacronyms[local,heading=none,preamble=subsection{Acronyms to Section #1}]
}

DeclareAcronym{ANN}{
  short = ANN ,
  long = Artificial Neural Network
}
DeclareAcronym{xx}{
  short = xx ,
  long = just a test
}
DeclareAcronym{yy}{
  short = yy ,
  long = another test
}

usepackage{etoolbox}
% make every section a barrier:
pretosection{acbarrier}

begin{document}

printacronyms[name=All Acronyms in the Document]

section{Foo}
subsection{Foo Subsection}
ac{xx} and ac{ANN}

printsectionacronyms{Foo}

section{Bar}
subsection{Bar Subsection}

ac{ANN} and text

printsectionacronyms{Bar}

section{Baz}
subsection{Baz Subsection}

ac{xx} and ac{yy}

printsectionacronyms{Baz}

end{document}