Unable to register zone as one already registered with that ID

[mjewild]

Since updating to 2.1.8 I'm seeing the following when calling java.time.ZoneId.of:

Exception in thread "main" java.lang.ExceptionInInitializerError
	at java.base/java.time.ZoneRegion.ofId(ZoneRegion.java:121)
	at java.base/java.time.ZoneId.of(ZoneId.java:411)
	at java.base/java.time.ZoneId.of(ZoneId.java:359)
	at Test.main(Test.java:9)
Caused by: java.time.zone.ZoneRulesException: Unable to register zone as one already registered with that ID: ical4j~e13c7b72-7f25-46ce-9ce9-e47946f19498, currently loading from provider: net.fortuna.ical4j.zoneinfo.outlook.OutlookZoneRulesProvider@4c1db628
	at java.base/java.time.zone.ZoneRulesProvider.registerProvider0(ZoneRulesProvider.java:324)
	at java.base/java.time.zone.ZoneRulesProvider.<clinit>(ZoneRulesProvider.java:191)
	... 4 more

This can be replicated with the following:

public static void main(final String[] args)
{
	ZoneId.of("Europe/London");
}

With org.mnode.ical4j:ical4j:4.0.0-rc3 declared on the classpath first, before org.mnode.ical4j:ical4j-zoneinfo-outlook:2.1.8
In maven the classpath order seems to come from the order the dependencies are declared.
The providers are then registered in the following order:

1. net.fortuna.ical4j.model.DefaultZoneRulesProvider
2. net.fortuna.ical4j.zoneinfo.outlook.OutlookZoneRulesProvider

The zone IDs for both come from net.fortuna.ical4j.model.DefaultZoneRulesProvider#provideZoneIds which is TimeZoneRegistry.ZONE_IDS.keySet()
i.e. it returns the same set of IDs twice

If the order of providers is reversed (OutlookZoneRulesProvider first) then the first time in provideZoneIds will return an empty Set as it hasn't been populated yet (done in the default constructor for DefaultZoneRulesProvider).
It therefore doesn't register the same set of IDs twice and doesn't fall over.