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Identical TLEs are often published by CSpOC at https://www.space-track.org/.
For example, here’s the first line of the three newest TLEs for IRNSS-1H:

1 42928U 17051A   19061.11119682  .11183803 -20635-4  33328-3 0  9993
1 42928U 17051A   19061.11119682  .11183803 -20635-4  33328-3 0  9993
1 42928U 17051A   19061.11119682  .09710361 -20650-4  26554-3 0  9995

All the TLEs have exactly the same epoch and the two newest TLEs are both identical.
Should I consider valid only the latest TLE?

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  • $\begingroup$ This kind of thing has always bothered me, also those with epochs only that differ by only microseconds or milliseconds. I suppose the salient question is if it makes any difference. After propagating them with SGP, how much do the resulting in state vectors differ from each other? $\endgroup$ – uhoh Mar 5 '19 at 23:14
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    $\begingroup$ For the TLE epoch, the difference is "only" 2.548 km and 5.56 m/s, but if the TLEs/SGP4 are used to calculate the initial state vector to be propagated with an high accuracy integrator, then it makes a lot of difference: after a few weeks, the result is totally different. $\endgroup$ – Cristiano Mar 5 '19 at 23:42
  • $\begingroup$ It's a problem for sure, but that's roughly the same as the difference between any one TLE and reality. I'm just speculating, but it may turn out that different TLEs issued for nearly the same epoch may end up being no more different from each other than they are different from the satellite's real trajectory. It might be good to find a pathological case, one where they differ so much that one is clearly "wronger" than the other. $\endgroup$ – uhoh Mar 6 '19 at 0:07

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