Shinzon was created twenty years before Nemesis to be a clone of Picard. Why? Why would the Romulans of twenty years before have known where Picard was going to end up in the future? He would have just been captain of the Stargazer at the time, just another average officer commanding an average ship.
The Romulans have two pieces of knowledge about future events.
One would be from interrogations and "mind sifting" of the survivors of the Enterprise-C (in particular Tasha Yar) in 2344, who could tell them about Picard, the Enterprise crew, the Enterprise-D, and the future strategic situation in the quadrant... with a twist: In this alternate universe, the Federation is much more militaristic due to being in a constant state of war and is on the verge of being conquered by the Klingons.
The Romulans might have based their whole strategy around this glimpse of what they didn't know was an alternate
future. They take an even more aggressive stance toward the Klingons, they return to the galactic stage, they build massive ships twice the size of the Enterprise-D for intimidation purposes, and they begin formulating a plan to replace high level captains (and possibly others) like Picard with clones.
The other glimpse of the future would be from Romulan captain Telek R'Mor in 2351 who made contact with the USS Voyager in the future through a micro-wormhole and could give them technical observations, scans of the ship, the crew's personal messages to their families and official reports to Starfleet, and vague details about the Federation's future.