When clicking on the double furnance, it shows "Unknown key: "recipe-name.double-iron" (Recipe)".

Seems to be cosmetic only. Is this a localisation bug? I play with the standard english factorio

More like a slight oversight. The recipe name is only shown in Factoriopedia, so no localization was ever done for it.
I'm not home so can't do a pull request for Raid but you can fix it by adding the below to lines to
<mod folder>\locale\en\double-furnace.cfg

[recipe-name]
double-iron=Steel Plate

When clicking on the double furnance, it shows "Unknown key: "recipe-name.double-iron" (Recipe)".

Seems to be cosmetic only. Is this a localisation bug? I play with the standard english factorio

Where do you see this? Can you provide a screenshot? I wasn't able to find the place your referring to, when I click on the double furnace it shows the correct strings for me in all UIs, so I'm not sure what I'm missing here 🤔

Use alt + left click to open the furnace in factoripedia. It lists all recipes a machine uses.

That is where it shows

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