The temporal definition in MonetDB says that:
"TIMESTAMP [ '(' posint ')' ] date concatenated with unique time, precision"
That this mean that if two rows are created with a DEFAULT TIMESTAMP that these timestamps are guaranteed to be unique, even if they are created during the same millisecond?
A MonetDB timestamp is (quoting from mtime.mal):
# @+ Timestamp
# An absolute point of time, as formed by the combination of a date a daytime in GMT
# e.g. 1999-01-31@@23:59:59:000
so, your answer is no, a timestamp is not guaranteed to be unique.
caveat: I haven't looked at the part of the MonetDB sources which handles DEFAULT TIMESTAMP, so I'm only 90% sure about my answer.
