Author a sibling block in case best block's slot is same as current slot

lib/babe/babe.go

@@ -497,6 +497,21 @@ func (b *Service) handleSlot(epoch, slotNum uint64,
 		return err
 	}
 
+	slotNumOfTheBestBlock, err := b.blockState.GetSlotForBlock(parentHeader.Hash())
+	if err != nil {
+		return fmt.Errorf("could not get slot for block %s: %w", parentHeader.Hash(), err)
+	}
+
+	if slotNumOfTheBestBlock == slotNum {
+		// pick parent of best block instead to handle slot
+		newParentHeader, err := b.blockState.GetHeader(parentHeader.ParentHash)
+		if err != nil {
+			return fmt.Errorf("could not get header for hash %s: %w", parentHeader.ParentHash, err)
+		}
+
+		parentHeader = newParentHeader
+	}
+
 	if parentHeader == nil {
 		return errNilParentHeader
 	}