Skip the same slot as in parent block

[nazar-pc]

Substrate already ensures to not claim the same slot twice, but it doesn't have any checks if the parent slot happens to be the same as current, so we need to check it ourselves.

Fixes #1328

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[liuchengxu]

Hmm, doesn't this line ensure the produced slots are always monotonically increasing?

https://github.com/paritytech/substrate/blob/41ae90843b070786ab53071479c141a7c6649ac9/client/consensus/slots/src/slots.rs#L174

[nazar-pc]

No, it only ensures that it claims increasing slots, if parent slot was produced by someone else, there is nothing preventing you from trying to claim the same slot. This is what I was trying to describe in the comment.

[liuchengxu]

if parent slot was produced by someone else

produced by someone else is the key, now I see, I was thinking from the single node context, but it could be from someone else, thanks for the explanation!