Extra Welfare servants?

Yes it's true, but we'll have that feature in two years more. Anyway if you have space maybe you can save the copies...

Keep them on the Second Archive, in 2 years you will be able to get rare Prisms (around the reruns of 3rd Christmass).

This is a big misunderstanding. You can't exchange/burn dupe copies of welfare servants for Rare Prism on JP.

The way it works is that each time you GET a copy of an specific welfare, that you have at NP5, on an specific rerun, they send a rare prism to your gift box. This means you will have the useless duplicate copies plus the rare prism of each copy.

They will never give you Rare Prisms for extra copies of Santa Alter, they only send rare prism when you get extra copies of Atilla the San during that specific rerun 2 years from now. It's similar to how the items for special summoning work, you know those wheel looking stuff that they give you each time you get a copy of a 5* servant you already have at NP5 (Just whale things).

We are typically not stuck to the JP timeline when it comes to quality of life changes, so I don't think it's fair to assume that we'd have to wait 2 years to start getting these rare prisms.

Either burn or use as exp