It is the milliseconds since midnight Jan 1, 1970.
Time is linear, just because at 2am some places decide they're going to now call it 1am, doesn't actually change how many milliseconds have passed. There aren't 2 day's a year that have 25 hours.
Are you sure the data is actually duplicated? Or is it possible that when the t_stamp is converted to a date the conversion is throwing things off?