CGT calculation when selling shareholding over a number of years

Is there an advice leaflet regarding the CGT treatment of the partial sale of a shareholding over more than one tax year? I'd have thought this a common experience, but couldn't find a thread on this by using a few related search terms. To be more precise by example. Suppose someone has a shareholding in a specific company (or ETF etc). They sell half the holding one tax year, calculate a capital gain on that and report it. They then sell the remainder of the shareholding the following tax year, but suppose that again generates a capital gain. If they then declare that full gain in year two, isn’t there a risk they are double-counting some of the gain previously reported? A worked example (say over two years) would be nice!

Hi taxiscomplex,
Please refer to:
Shares and Capital Gains Tax (Self Assessment helpsheet HS284)
And the link for example 3
Thank you.