More clear error message: "Duplicate entry for markerID=..."

When a height file contained a duplicate entry the following error message was printed to the log: ´Duplicate entry for markerID=...´

(PeTrack crashes, with this occurence)

Suggestion: Change to ´Duplicate entry found in height file for markerID=...´ (+ ´PeTrack does not know what to proceed with´?)

For discussion:

• how about changing code to automatically take the first entry and printing a warning instead, in order for petrack not to crash?