更改此代码

    $sqlin="INSERT INTO data_test (username, password) VALUES ('valname','valpass')";
    $result=$conn->query($sqlin);
    if ($conn->query($sqlin) === TRUE) {
    echo "New record created successfully";
    } else {
     echo "Error: " . $sqlin . "<br>" . $conn->error;
     }
     $conn->close();
     include_once "read.php";

到

$sqlin="INSERT INTO data_test (username, password) VALUES ('valname','valpass')";
$result=$conn->query($sqlin);
if ($result === TRUE) {
echo "New record created successfully";
} else {
 echo "Error: " . $sqlin . "<br>" . $conn->error;
 }
 $conn->close();
 include_once "read.php";

因为如果您在此行$conn->query($sqlin);中运行两次，它将运行两次查询并两次在数据库中插入记录

Because if you run this line $conn->query($sqlin); twice it will run your query twice and insert record in database two time