Here is a record of the unixtime printed every .2 seconds during the leap second. Notice that 1230767999 repeats for two seconds.

1230767998.47129
1230767998.67142
1230767998.87153
1230767999.07163
1230767999.27177
1230767999.47188
1230767999.67198
1230767999.87212
1230767999.07224
1230767999.27234
1230767999.47244
1230767999.67255
1230767999.87267
1230768000.0728
1230768000.27291