Concatenate multiple files with same headers (and only keep one header line in the output file) - Version2

 Put this into an AWK script and call it merge.awk:

BEGIN{

    h=ARGV[1];

    ARGV[1]="";

}

{

    if(FNR==1){print}

    else if($1!~h){print}

}

and use an argument:

cat *.csv | awk -f merge.awk "<header>" > output.csv