if(ver se existe no banco){

.....if(){

..........faz insert
.....}

}else{

.... faz updade

}

if(!empty($_POST['arrachar'])){
    // Loop to store and display values of individual checked checkbox.
    foreach($_POST['arrachar'] as $selected){
$string = implode(',', $selected);

preg_match('/(d{4}-d{2}-d{2})/', $string, $dia);
$dia = $dia[0];
if($sq="SELECT arrachar FROM centrodb.marcacao WHERE arrachar like '%".$dia."%'"){
if(preg_match('/(d{4}-d{2}-d{2})/', $string)){
$sql="INSERT INTO marcacao (arrachar) VALUES('$string')";
$r = mysqli_query($conn,$sql);

}
    }else{ $sql1="UPDATE centrodb.marcacao SET arrachar = '$string' WHERE arrachar = '%".$dia."%'")
     $rs = mysqli_query($conn,$sql1);
}
    }
}

if($sq="SELECT arrachar FROM centrodb.marcacao WHERE arrachar like '%".$dia."%'"){
....
}else{ $sql1="UPDATE centrodb.marcacao SET arrachar = '$string' WHERE arrachar = '%".$dia."%'")
     $rs = mysqli_query($conn,$sql1);
}