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Some additional examples
User logged in as JanEl
I think King George III of Great Britain ("Mad George") might count, as he was eventually deemed too mentally unfit to serve as sovereign -- although by his day, British kings no longer had ruling power. A regency was declared in 1811, with his eldest son and heir-apparent (the future King George IV) taking over his father's sovereign duties and functions. The Prince of Wales now bore the title Prince Regent of the UK.
As for Duke Enrico of Bourbon-Parma, who in 1907 succeeded his father (Roberto) as head of the house, a regency was declared for both him and his titular successor, Duke Giuseppe. It was assumed by their brother (Duke Elia), for reasons of mental incapacity. But by then, the duchy of Parma had long been deposed; so I'm not sure if these examples count. One is not talking of active sovereignty, here ...