Fix list concat overwriting

[peterebden]

Fixes #3166

Looks like this is due to the slices overwriting since on append they both think they have extra capacity, but are in fact sharing it. A pure Go example: https://go.dev/play/p/pLl6MlMr3V4

This fixes this particular case; I don't think there are others at present, we seem to construct lists with capacity equal to their final size in other instances.