Act as an experienced SEO specialist with 5+ years of technical optimization expertise. Write a detailed 800-word guide explaining the impact of canonical tags on SEO, covering: [1] how search engines interpret canonical tags, [2] common implementation mistakes (e.g., [SELF-REFERENTIAL] vs [CROSS-DOMAIN] usage), and [3] case studies showing [POSITIVE/NEGATIVE] ranking effects. Include best practices for [E-COMMERCE] sites with duplicate product pages. Use markdown formatting with H2/H3 headers, bullet points, and a comparison table of rel=canonical vs 301 redirects.