Both sprockets and the chain need replacing.
150mm is the maximum serviceable stretch tolerance, yours is 153, the front sprocket is hooked, the rear is worn. Many owners make the big mistake of not changing sprockets when the chain is replaced, it's bad practice and false economy, worn sprockets will half the chain's service life.
I've seen all manner of cons, rip off's and blatant fraud by stealers and owners over the years. Everything from fitting parts from written off bikes with new or newer chains/sprockets and passing it off as new, to not changing front sprockets as many owners never take the front cover off. Dual links although not common, isn't new, there're all manner of reasons this could have been done. The most common a stealer or owner uses old chains and builds one out of 2, chains when replaced only have a few tight links, they remove them and rejoin, so as there's no tight links, this doesn't remove the worn ones! Why cost, stealer pockets the whole cost of the new chain while supplying one at not cost, owners fit them just before selling saving a wedge and not spending on a bike they're about to sell.
Just replace both the sprockets and the chain.
I would recommend you go over the whole bike and look for more of this kinda of thing, brake pads, air filters, are the tyres a matched pair, what make are they etc, etc. The rims looked to have been polished, the bike from the areas in your pictures isn't in the condition I would expect from 98 very low mileage bike, mine is 98 it's covered 57k and is much cleaner in those areas than yours. I'm suspecting the mileage, hard to be certain without more pictures, of discs, bolts, screws, at 24k the latter should show few signs of wear/use 24k is only 3 services!