What does this mean?

All ID attributes on a page must be unique, or accessibility technology used by the page can stop working.

For example, the duplicate id here makes it unclear which label belongs to which input field:

<label for="name">Your name</label>
<input id="name">

<label for="name">Company name</label>
<input id="name">

Similar issues can occur when using aria-labelledby and aria-describedby.

To fix this, ensure all id attributes on a page are unique.

See Technique H93 for more information.

76.7% done

How close this website is to fixing this issue.

HTML Found on page Issues
<div class="contact postal-address" id="contact_3564">...</div> 7
<div class="contact postal-address" id="contact_3567">...</div> 2
9 distinct issues were found in the sample of 14 web pages.
More results from Forestry Commission (GOV.UK)