The only reason I can see for getting a second watch of a model that you already own would be if you were scared about losing or damaging the one you have and feel you need a spare.

Personally while I can understand the desire to have backups, in this case it is overkill. As a worst case, your watch gets irreparably damaged in a few years time - even if they are not available new, most standard production Seiko’s are still going to be available. Even if you have to pay a bit of a premium for a good example it will not be impossible to get.

That also assumes your tastes are still exactly the same and Seiko have completely dispensed with your chosen watch range. Buying a spare locks you in to that model, while waiting allows you the opportunity to revise your opinion and buy something different that may have been released.

So I wouldn’t worry about keeping a spare to hand. Spend your money on something more immediately useful and a replacement watch can be figured out in the future if it is ever necessary.