Validate duplicated StopPointInJourneyPattern ids

[vpaturet]

Duplicated StopPointInJourneyPattern ids should be rejected:

<StopPointInJourneyPattern order="1" version="20251125092607" id="SKY:StopPointInJourneyPattern:1cef7911-10a4-445e-9e65-4f85124a01a0">
  <ScheduledStopPointRef ref="SKY:ScheduledStopPoint:1cef7911-10a4-445e-9e65-4f85124a01a0"/>
  <ForAlighting>false</ForAlighting>
  <ForBoarding>true</ForBoarding>
  <DestinationDisplayRef ref="SKY:DestinationDisplay:1cef7911-10a4-445e-9e65-4f85124a01a0"/>
</StopPointInJourneyPattern>
<StopPointInJourneyPattern order="2" version="20251125092607" id="SKY:StopPointInJourneyPattern:1cef7911-10a4-445e-9e65-4f85124a01a0">
  <ScheduledStopPointRef ref="SKY:ScheduledStopPoint:1cef7911-10a4-445e-9e65-4f85124a01a0"/>
  <ForAlighting>true</ForAlighting>
  <ForBoarding>false</ForBoarding>
</StopPointInJourneyPattern>

Similar to these issue in OTP:
opentripplanner/OpenTripPlanner#7092