Upload form duplicate file in mysql when I refresh browser - mysql

Question

I have a problem with refreshing upload file form. First time when I clik Upload File it uploads file on server (folder uploads/), and path name uploads in mysql base (table 'files'). When I refresh that page it duplicates same path name in mysql base, it does not duplicate file in uploads/. I found similar solutions but not my case, where I need to stay on the same page, and list all uploaded files. Here is the form code :
<?php
if(isset($_POST['upload'])){
$targetfolder = "uploads/";
$target_file = $targetfolder . basename( $_FILES['file']['name']) ;
$ok=1;
$file_name = $_FILES['file']['name'];
$file_type=$_FILES['file']['type'];
if ($file_type=="application/pdf" || $file_type=="image/gif" || $file_type=="image/jpeg" || $file_type== "application/msword" || $file_type == "application/vnd.openxmlformats-officedocument.wordprocessingml.document" ) {
if ($ok == 0) {
echo "Sorry, your file was not uploaded.";
} else {
if (move_uploaded_file($_FILES["file"]["tmp_name"], $target_file)) {
$sql = "INSERT INTO files (path, profesor, struka) VALUES ('" .$file_name. "', '". $_SESSION['username']. "','')";
if (mysqli_query($link, $sql)){
}else {echo "ERROR connecting database $sql".mysqli_error($link);
}
} else {
echo "Sorry, there was an error uploading your file.";
}
}}
else {
echo "Sorry only PDF, DOCX, JPEG, GIF.<br>";
}}
?>
<div class = "upload">
<form method="post" enctype="multipart/form-data" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
Select file for upload:</br>
<input type="file" name="file" >
<input type="submit" id = "button_upload" value="Upload File" name="upload">
</form>
</div>

Answer 1

first, i don't see a place in your code, where you actually show the list of uploaded files.
second - if you refresh your page, the form is submited one more time, so another insert is done with the same data. you don't see the duplicate file, because it is just overwritten.
so, after you successfully inserted and uploaded, you should do a redirect to the same page, for example with header('Location: yourscript.php'). in this case POST will not be set.