Let’s kill the myth first: Google has no “duplicate content penalty” for ordinary sites. What exists is quieter and still costly — dilution: when several of your URLs say the same thing, Google picks one (maybe not your favorite), splits signals among them, and ranks the muddle lower than one clear page would rank.
Where Czech sites actually hit it
Product variants each getting a URL; print versions and session parameters; the www/non-www and http/https twins both serving; category descriptions copy-pasted across e-shop sections; and manufacturer descriptions duplicated across every reseller — the commonest e-commerce dilution, where rewriting is the entire fix.
The canonical: your official version
A canonical tag says “of these similar pages, this is the one” — consolidating the split signals. Most WordPress SEO plugins set self-canonicals automatically; the failures come from misconfiguration and migrations, which is why audits check canonicals early.
Same-content-different-language is not duplication, but needs hreflang — a different tag for a different job. Suspect your pages compete with each other? The Performance report shows it in minutes.
Frequently Asked Questions
Not for ordinary sites — the real cost is dilution: your own URLs splitting signals for one search.
Your declaration of the official version among similar pages — it consolidates split ranking signals.
No — language versions need hreflang, not canonicals; different tag, different job.
Manufacturer descriptions repeated across every reseller — rewriting them is the entire fix.
Search Console's Performance report shows multiple URLs competing for one query in minutes.
If both serve, yes — redirect one to the other and pick a single official form.
English 
Czech