Baker: duplicates included in quorums

The following issue was observed for the tezt test 'baker forked migration blocks from mumbai to alpha':

(Copying from !7957 (merged))

The baker output in this job suggests a serious problem in the baker: some consensus operations are counted twice!

• https://gitlab.com/tezos/tezos/-/jobs/3850456476#L568: bakers b1 to b4 (but not b5) preendorse, and then bakers b1 and b2 endorse even if there is no preendorsement quorum; there is no quorum because b5 does not preendorse and the consensus threshold is set to 100%
• https://gitlab.com/tezos/tezos/-/jobs/3850456476#L794: an explicit error showing the problem

This double counting was confirmed with !7964 (closed), see https://gitlab.com/tezos/tezos/-/jobs/3905089850 (for Alpha) and https://gitlab.com/tezos/tezos/-/jobs/3909928542 (for Mumbai).