Here are the nine reasons he cited for canonical choices:

Exact duplicate content
The pages are fully identical, leaving no meaningful signal to distinguish one URL from another.
Substantial duplication in main content
A large portion of the primary content overlaps across pages, such as the same article appearing in multiple places.
Too little unique main content relative to template content
The page’s unique content is minimal, so repeated elements like navigation, menus, or layout dominate and make pages appear effectively the same.
URL parameter patterns inferred as duplicates
When multiple parameterized URLs are known to return the same content, Google may generalize that pattern and treat similar parameter variations as duplicates.
Mobile version used for comparison
Google may evaluate the mobile version instead of the desktop version, which can lead to duplication assessments that differ from what is manually checked.
Googlebot-visible version used for evaluation
Canonical decisions are based on what Googlebot actually receives, not necessarily what users see.
Serving Googlebot alternate or non-content pages
If Googlebot is shown bot challenges, pseudo-error pages, or other generic responses, those may match previously seen content and be treated as duplicates.
Failure to render JavaScript content
When Google cannot render the page, it may rely on the base HTML shell, which can be identical across pages and trigger duplication.
Ambiguity or misclassification in the system
In some cases, a URL may be treated as duplicate simply because it appears “misplaced” or due to limitations in how the system interprets similarity.