// Create an array
a = new Array();

// Fill the array a with all lines (with returns) in the document.
document.selection.StartOfDocument();
for( ; ; ){
    y = document.selection.GetActivePointY( eePosLogical );
    document.selection.SelectLine();
    sLine = document.selection.Text;
    if( sLine == "" ) {   // Reached the end of document, escape from the loop
        break;
    }
    a.push( sLine );
    document.selection.Collapse();
    if( document.selection.GetActivePointY( eePosLogical ) == y ) {
        // Reached the end of document (the last line without return), escape from the loop
        break;
    }
}

// Delete duplicate elements.
for( i = 0; i < a.length; i++ ){
    sLine = a[i];
    for( j = i + 1; j < a.length; j++ ){
        if( sLine == a[j] ){
            a.splice( j, 1 );
            j--;
        }
    }
}

// Replace the entire document with new elements
document.selection.SelectAll();
document.selection.Text = a.join( "" );