or have there already been

Both. We get into the numbering conventions of British monarchs. William the Lion was king of Scotland 1165-1214. he was the only William who was solely king of Scotland.

Meanwhile in England we have William I (1066-1087) and William II (1087-1100).

Cut to 1603, when James VI of Scotland inherits the throne of England and becomes James I. Sometimes he is styled "James VI and I", but usually we just assume England outranks Scotland and call him James I.

His next two successors were called Charles, and as neither country had had a Charles before, they're unproblematically Charles I and Charles II. Then comes James II, as he's usually called, or who could pedantically be called James II and VII. He is deposed in 1688 and the throne of both countries is given to William and Mary. Luckily both countries have already had one Mary, so she's Mary II of both. He is always called William III, never William III and II.

In 1707 the two countries were united under Anne, who became first monarch of Great Britain. Neither old kingdom had had an Anne, George, or Victoria before, so these didn't present a problem. When another William came the throne in 1830 he became William IV, and when Edward acceded in 1901 he became Edward VII, both taking the English regnal number, which was also the higher one.

In about 1910 a new rule was adopted, that where there was a difference in regnal numbers, the higher number would be used. So the next James will become James VIII, and if there's a Robert he'll be Robert IV.