Given that the table had no index at all, I didn't think in this case that making the clus index unique was necessary, particularly to the extent of adding the overhead of an identity column. SQL can very quickly seek to find the highest key value whether it's unique or not.
But, to help cut down on dup key values, make the underlying column datetime2 rather than datetime.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial: "If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them."