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My potato plants have produced fruit, which are a bit like hard green tomatoes, is this normal? Please forgive my ignorance.

I don't know, first my Garlic has bulbils now my spuds have berries!

Yes, it's perfectly normal.
Not all potato varieties do it, but most do. It's the seed pod and if you have the patience you could grow some and see what they turn out like. They will be hybrids and possibly nothing like the parent, but you would be able to name the "new" variety.
Don't eat them though as they are poisonous.