What epoch to use for kwhd inserts

I want to import some old kwhd data into a newly generated feed. But somehow I keep getting double entries in the bargraph. My system and emoncms are both on the same timezone (europe/amsterdam), and I tried now with both the 00:00:00 epoch from gmt and my timezone, but the double entries stay. Any insights on what epoch to use to get rid of this?

An insert string looks like;
Where the epoch is GMT: Sat, 26 Apr 2014 00:00:00