German photographer Martin Schoeller's photographs of twins challenge our perceptions of "sameness".
He states: "Questioning the principles that define the sameness and differences of identical siblings is a study of the core habits of perception—the extent to which our environment impacts our development and our expectations, our preferences, and our choices. We assemble impressions of and form mysterious attachments to universal features—hairline, forehead, eyes, nose, mouth, and chin. We take their arrangements to be unique. Identical twins dispute that assumption."
This series can be found in the book "Identical: Portraits of Twins". I love how thought-provoking this work is, and its straightforward pictorial approach to the subject.