Adding milliseconds to zoo object

Thank you for replies Gabor & Brian,

Yes I did read the previous threads concerning similar problem and I saw
Dirk's post:

I pad my high-resolution times with an eps=1.0-6 to make them distinct,

if

need be 

So I presume the question is better formulated as how to pad high-resolution
times with an eps to make them distinct given that we have original index
with non-unique values, but order in the file reflect their relative
position inside this time interval.
 
2007-12-24 06:14:00  22.00
2007-12-24 06:14:00  22.00
2007-12-24 06:15:00  22.00
2007-12-24 06:15:00  22.00

My current idea is to convert timestamps to factors and run tapply on them,
but probably there is a better solution.