timestamp precision

Hello,

Im using the default precision for my timestamps, 6.  Is it safe to
declare this column unique?  Because you can define the level of
precision I assume you could theoretically have a duplicate.  Or does a
precision of 6 give you an accuracy at the point where pgsql could never
do 2 transactions in the given timestamp time span?

I likely could have written this better, I hope its understandable.

Thank you in advance.

A Gilmore