为什么我的PHP代码INSERT命令有时会插入重复数据？

Here is My Code. Its all working fine. But Some Times it inserting Duplicate Data. Why? I mean i placed the code so that same quize_no and phone combination should not submitted. But Some time its Inserting the Same quize_no and phone combination. Why is this Happening?

$sql = mysqli_query($con,"SELECT * FROM user_mm WHERE phone='$phone'");
while($row = mysqli_fetch_array($sql)) {
    $name = $row['name'];
}

date_default_timezone_set("Asia/Dhaka");
$date = date('Y-m-d');
$time = date('h:i:s a', time());

$result = mysqli_query($con,"SELECT id FROM score_mm WHERE phone='$phone' AND quize_no='$quize_no' ");

$count=mysqli_num_rows($result);
if($count>0)
{
    echo "1";
}
else
{
    // Here I am Inserting Value in to Database
    $query = mysqli_query($con,"insert into score_mm(name, phone, quize_no, score, date, time) values ('$name', '$phone','$quize_no', '$score', '$date', '$time')");
    echo "Score Submitted Succesfully";
    mysqli_close($con); 
}

If there should truly never be a situation where you end up with duplicate quize_no and phone combinations in that table, then the first thing you should do is change your table structure to add a unique key to those columns.

For example: ALTER TABLE score_mm ADD UNIQUE (quize_no, phone);

This won't fix whatever is causing multiple insert requests to be called, but it would at least protect your data integrity by not allowing any rows with duplicate value combinations.