Duplicate timestamps

[pwoods25443]

Duplicate timestamps are a problem. Because you can't compute a speed, we often end up creating a new segment if the lat/lon is not identical between two messages with duplicate timestamps within the same ssvid of course)

One solution is to just toss out the duplicates, keeping only the first one. This is crude, but it solves the problem. Probably what we should do is say that a duplicate timestamp can never cause a new segment to be created, but it can be matched to an existing segment if it is within a small distance.