Google On Difference Between Clustering & Canonicalization
Briefly

Allan explained that people often perceive canonicalization as a magic black box, when in reality it is just one step in a larger process that includes clustering.
When addressing complaints about canonicalization issues, Google's first response typically points to clustering problems, indicating that not all pages belong in the same group.
John Mueller succinctly clarified that clustering refers to grouping similar pages, while canonicalization is the process of deciding which page from that group is the most relevant.
Read at Search Engine Roundtable
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