1761 Susannah Trevelyan by Thomas Gainsborough and over-painted by Sir Joshua Reynolds (Wallington Hall - Northumberland) Photo - Nicky Grimaldi
Nicky Grimaldi, senior lecturer in conservation and fine art at Northumbria University, found that Gainsborough started this and completed it, with a different dress in ultramarine. The owner was offended when the original was dismissed as a "hat and ruffles" portrait. The owner brought in Reynolds who redid up to 80% of the original, making it a clone of the Charlotte Walpole portrait in the process.
Susannah Trevelyan's article in thepeerage is here.