Avoid duplicates in MYSQL database?

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PHP

Ajax

Hi
How to avoid inserting duplicate values into a database?

Below code inserts into database, but I don't want separate rows for example, have User ID with name alan and another row saying a different name, i want it to be userID and names associated in one

Can someone help?

What I have tried:

try{
	
$stmt = $conn->prepare('INSERT INTO favourites (nameID,UserID) VALUES (:nameID, :UserID);');
			$stmt->execute(array(
				':nameID' =>$_POST['nameID'],
				':UserID' =>$_POST['UserID']
				));
			$count = $stmt->rowCount();
			if($count > 0 )
			{
				echo "<span style='font-size:100%;color:red;'>❤️";
			}
			else {
				echo "unsuccessful";
			}
}
catch(PDOException $error)
{
	echo $error->getMessage();
}
?>