Insert and update statement for duplicate rows

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SQL

PHP

MySQL

I have a table postviews which has columns post_id and ipaddress . I use a INSERT statement for this table on posts page whenever a post is opened, id of that post and IP address of viewer are sent to database and saved in their respective columns.

$stmt =$db->prepare("INSERT INTO postviews (post_id,IPaddress) VALUES (:post_id,:IPaddress)");

Now whenever one user opens same post again and again new rows are inserted with same post id and IP address which I want to prevent. So now I have added a third column counter to this table, so now whenever one user opens same post again and again instead of inserting new rows with same post id and IP address I would rather like to increase my counter column. I am not sure how to do it. Will something like this work:

$stmt =$db->prepare("INSERT INTO postviews (post_id,IPaddress) VALUES (:post_id,:IPaddress)
ON DUPLICATE KEY UPDATE counter = counter + 1");

I want to prevent creating new row only when both columns (post id and IP address) are same , but a new row should be created if any one of them is different than previous row, so unique composite key couldn't be used here.