The claim
When multiple URLs serve similar content, Google consolidates them and indexes one canonical, so unmanaged duplication dilutes and misdirects ranking signals.
What the evidence shows
Google's documentation on canonicalisation and consolidating duplicate URLs describes clustering near-duplicate pages and choosing a canonical based on signals including canonical tags, redirects, internal links and sitemaps. Left unmanaged, Google may pick an unintended canonical, splitting authority and suppressing the preferred page.