You can use make.index.unique
in the xts package.
x <- structure(list(value = c(18, 62, 64, 29, 22),
date = structure(c(1373406563, 1373410896, 1373413611, 1373413611, 1373413611),
class = c("POSIXct", "POSIXt"), tzone = "")), .Names = c("value", "date"),
row.names = c(NA, -5L), class = "data.frame")
x$date.unique <- make.index.unique(x$date,1)
x
# value date date.unique
# 1 18 2013-07-09 16:49:23 2013-07-09 16:49:23
# 2 62 2013-07-09 18:01:36 2013-07-09 18:01:36
# 3 64 2013-07-09 18:46:51 2013-07-09 18:46:51
# 4 29 2013-07-09 18:46:51 2013-07-09 18:46:52
# 5 22 2013-07-09 18:46:51 2013-07-09 18:46:53