Original: The only individual to have suffered such a fate was John Erskine, 6th Earl of Mar who lost both the knighthood and the earldom after participating in the Jacobite rising of 1715.
Simplified: But this has happened only once, to John Erskine, 6th Earl of Mar who was deprived of  both the knighthood and the earldom after taking part in the Jacobite rising of 1715.