Is duplicate content a penalty?

No, not in ordinary cases. There is no duplicate content penalty. The cost is that ranking signals get split across URLs and the engine picks a canonical version that may not be yours — which is a competition problem rather than a punishment.

Do I need an XML sitemap?

Yes, and it should be generated rather than hand-written. A sitemap helps engines discover URLs they might otherwise miss and signals when pages genuinely changed. It cannot make a thin page rank, and it will not rescue a page nothing links to.

What is JSON-LD and why is it preferred?

JSON-LD is a block of JSON inside a script tag describing what a page is about. Google recommends it over Microdata and RDFa because it sits apart from the visible markup, so a design change cannot silently break it.

SEO

What is a canonical tag for?

A canonical tag tells search engines which URL is the preferred version when the same content is reachable at several addresses. It consolidates ranking signals onto the URL you nominate, rather than letting the engine choose for you.

Answered by Anas Bin Masud 175 words 3 sections

Why almost every site needs them

Sites serve the same content at multiple URLs without meaning to: with and without a trailing slash, with tracking parameters, through two category paths, in a print view. Left alone, the engine picks one, and often not the one you would have chosen.

The rules

Every indexable page carries a self-referential canonical
Absolute URLs, not relative
Filtered and sorted views canonicalise to the clean category URL
Never canonicalise to something that is not genuinely equivalent — it gets ignored
A canonicalised URL should not appear in your sitemap

The failure that takes months to notice

Every page canonicalising to the home page, usually through a template variable that was never set per page. Nothing else is treated as worth indexing, and the pages still exist and still respond, so nobody looks.

The longer version

This answer is deliberately short. If you want the full treatment, Technical SEO, in the order the problems actually block you covers it in about 4 minutes — free, ungated, written from client work.

Who answered this

Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan — fifteen live client sites, six of them stores taking real payments. These answers come from those projects rather than from a content brief. More about how I work, or ask me something directly.