CONTAINS SPOILERS AND MEANINGLESS JIGGING ABOUT
Case for the defence: I am not, and never have been, an S Club fan. I know precisely zero of their songs and am only aware who Hannah Spearitt because she turned up in ITV's fun dinosaur show Primeval, and I honestly wouldn't know the others if I sat next to them on a bus. The only reasons I have this film on DVD are  the scriptwriters were Kim Fuller and Paul Alexander, who contributed to the disastrous seventh series of Red Dwarf, and  it was on sale for 10p in CEX. That's my story. And in fact, a mere two days after having watched the rampant intergalactic silliness of Seeing Double, I still couldn't name any of their songs because it's literally in one ear and out the other. The music does absolutely nothing for me, but that's the case for most modern pop music; it's just not my thing.
Obviously there's a comparison to be made with Spice World: The Movie, a film that again probably depends more than anything else on whether you like the band. And again, for me they're not good enough for me to go "Wow!" and not awful enough for me to go "Aaargh!"; they're just there. Seeing Double at least has more of a plot than Spice World, albeit a particularly bonkers one: a mad scientist wants to take over the world by cloning pop stars to capture the global teenage market. Bizarrely they've cloned S Club while still leaving the originals alive; they're promptly arrested and locked up because apparently pretending you're in S Club is a criminal offence. They escape, make it back to Los Angeles to find out what's going on and to confront their navel-less duplicates....
There are several songs, dance numbers involving a lot of jigging about and leaping up and down, one irresistible moment when the "real" Hannah wonders whether they are actually the clones rather than the originals, a castle full of lookalikes (apparently including Gareth Gates who  I didn't recognise and  actually plays himself) and fewer scenes of S Club interacting with their other selves than you might expect - you never really get to See Double. It's a shiny plastic diversion full of pretty people and pretty colours, 87 minutes of harmless and inoffensive silliness that does not linger in the memory. My only real problem is that CEX aren't going to buy it back if it only cost 10p, so one of the local charity shops will eventually find it in their donations box. Lucky them.
Or you could buy it here: