Timestamps in a channel must be uniqiue. Once written, you cannot edit a feed. A feed is the collection of a timestamp and the associated fields at that timestamp. You are trying to write data with the same timestamp as existing data to the channel. You would need to clear the channel to be able to write that data again. At the time that you write the first data point, you can simultaneously write all 8 fields, but not after you have written the feed.