As the title says, I have provided a key to every component and I believe they are unique. To reproduce this as simply as possible, I'm providing a snapshot of the rendered components with IDs that show no duplicates or absentees. This is taken from the react dev console and shows that keys are being picked up correctly for each component.
The second Step is throwing the following error:
Warning: Each child in a list should have a unique "key" prop.
I'm not having an functional issues but I do want to make sure I'm not doing anything wrong.
The only luck I've had so far is when removing the Marker component. The error disappears. However, all the sub levels have keys too, including Marker, so I can't make sense of this outcome.
The answer was something I'd overlooked:
<Step>
<StepPropGroup />
<StepPropGroup />
<React.Fragment>
<Marker />
<StepPropComment />
</React.Fragment>
</Step>
React.Fragments can also have keys. I was so unaware of this that I didn't even think of mentioning it as it was superfluous. However, adding a key to it now resolves the error.