From BBC News:

The Duchess of York has been diagnosed with malignant melanoma following the removal of a cancerous mole during treatment for breast cancer.

Sarah Ferguson had several moles removed and analysed while having reconstructive surgery following a mastectomy, her spokesman said.
The duchess “remains in good spirits”, despite it being “distressing” to have another cancer diagnosis.
She is the third royal to announce a medical procedure this week.

BBC: Sarah Ferguson: Duchess of York diagnosed with skin cancer