One of a Kind vs. The Only One of Its Kind

“One of a kind” and “the only one of its kind” are both expressions that are used to describe something unique. (Specifically, if you describe something as “the only one of its kind,” you’re emphasizing that it has qualities that put it in a category of its own; it is unique in a fundamental way.) They should not be combined into “the only one of a kind,” which is an awkward and redundant expression.

This painting is the only one of a kind.

This painting is truly one of a kind.
This painting is a one-of-a-kind work of art.

This ancient cave painting is the only one of its kind; archaeologists have found nothing else like it.