Assuming you already have overrides for "item.php" you can check if the K2 image propery exists (in each K2 item object coming from the related items loop - if (isset($item->image)) { ... }) and then either display a placeholder image or just use a different HTML structure.

So edit this line github.com/getk2/k2/blob/master/components/com_k2/templates/default/item.php#L474 from:
to:

Also fixed in K2: github.com/getk2/k2/commit/af51b22b11112db5e68e0d4657dada06e59ab9f4

This will be part of K2 v2.10.4.

Forgot to mention that the title you see as duplicate is actually the "alt" text from the <img> tag. With the new check, this won't be visible obviously.

Any chance some K2 images exist and some got deleted? K2 will check if the "Generic" size exists to determine whether K2 images are there for some item to use.