Duplicate IDs

I note the plunging is creating duplicate HTML IDs, which is not valid.

for example, take a look at my-calendar, via W3C validator.

https://validator.w3.org/nu/?doc=https%3A%2F%2Ffriendsofwollatonpark.org.uk%2Fmy-calendar%2F

The page I need help with: [log in to see the link]