approval-voting: Make importing of duplicate assignment idempotent (#6971)

Normally, approval-voting wouldn't receive duplicate assignments because
approval-distribution makes sure of it, however in the situation where
we restart we might receive the same assignment again and since
approval-voting already persisted it we will end up inserting it twice
in `ApprovalEntry.tranches.assignments` because that's an array.

Fix this by making sure duplicate assignments are a noop if the
validator already had an assignment imported at the same tranche.

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Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
Co-authored-by: ordian <write@reusable.software>