While this doesn't change the essence of your claim, the problem is not with UTC timestamps but with Unix timestamps. UTC timestamps (HH:MM:SS) are actually unique because the extra second inserted is assigned the numeral 60. Unix timestamps do, however, repeat when a leap second is inserted.
saltcured|1 year ago
A timestamp "repeats" to add a second (positive leap second). The subsequent timestamp sequence is delayed relative to its prior offset to TIA. The Google "smear" method works here to slow down the clock rather than repeat values.
To drop a second (negative leap second) we have a one second gap in the timestamp sequence. The subsequent timestamp sequence is advanced relative to its prior offset to TIA. This just requires a monotonic jump, as if the computer froze and did not perform any work for one second before resuming with the right clock values.
binarycoffee|1 year ago