Trigger lambda only on S3 object create, but not to trigger on override

There's no particularly easy way to do what you want to do here. PUT/POST events both send out events for object creation even when they are replacing existing objects.

You could enable versioning on the bucket and detect (by looking at the object metadata) whether there is a new version that has just been uploaded. But it would mean that you'd have extra copies of objects in the buckets. Those could be deleted using lifecycle rules but it's extra work to do.

Even more work would be to maintain a database which holds a list of existing objects; the Lambda function would check to see if this is a replace operation rather than a "proper" creation and stop running as appropriate.

That said, if you already have a database (or list or something) which you're trying not to populate with additional data when the objects are replaced then maybe you could query that? If the object doesn't exist in that store then it is new; otherwise it is a replacement.