You have to look after the spares really well.
It’s the ultimate end-point of “keep your friends close, but your enemies closer”, for your spare is both your best friend, and your worst enemy. And it can flip in seconds.
The real Queen knows it. She knows that the spares have to be kept in gilded luxury, entertained, plied with the finest wines known to humanity and serviced daily by handsome young X-factor contestants. Because the moment you have a disgruntled spare is the moment your reign is in peril. A furious spare only has to think “what if I was to pinch the crown and tell a passing Grenadier Guardsman that that stingey bitch isn’t the real Queen?”. The real Queen doesn’t want to find herself rotting in the Tower of London while Prince Philip makes mildly racist remarks to spare #2 over breakfast at Balmoral. She doesn’t want to fester in chains while spare #2 takes leisurely rides to Wolverhampton in the Royal Train, or asks people whether they’ve come far at sun-kissed garden parties. She doesn’t want to miss the weekly meeting with David Cameron and the annual knighting of new party donors. She loves the life, loves her country, and she fears her spares.
In fact, since the Civil List was cut down, the real Queen lives more frugally than the spares do. The real Queen has had to cut back, like a later season of Downton Abbey. But the spares are living it up. Because the real Queen fears them.