Do I really have to rip it to pieces?

Please see the golden rule! The point is to actually, literally rip the card up, so it will never be playable again. It is banished after you rip it.

How does this work in a tournament, especially in best of 1 games, when I rip this?

See the rules for that specific tournament, but by default the card is playable. Tournament organizers have latitude to set rules for their events, but one reasonable approach is that you use a deck missing one card for the rest of the tournament (and it's legal to do so).

If my opponent gains control of my Erik's Curiosa and then they rip it, who gets to pick the card?

They get a card of the controller's choice from the controller's collection (so they get to pick). Hopefully you can still be friends! (They must give you a moment to concede before they rip it.)

Does this effect let me break the normal restrictions on deck legality? e.g. duplicate Unique card, or duplicate any site for Pathfinder?

The card limits exist only when the game starts; once the game begins, various effects can change your deck composition. So, yes, you are free to get an extra copy of a Unique card via Erik's Curiosa, as long as the card is actually in your collection.

Erik's Curiosa

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