I get an IllegalStateException "<window> is in more than one state address
window set" (stacktrace below).
What does this mean? What invariant of custom window implementation
& merging am I violating?
Thank you for any advise.
```
java.lang.IllegalStateException:
{[2019-12-05T01:36:48.870Z..2019-12-05T01:36:48.871Z),terminal} is in more
than one state address window set
at
org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkState
(Preconditions.java:588)
at
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.MergingActiveWindowSet.checkInvariants
(MergingActiveWindowSet.java:334)
at
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.MergingActiveWindowSet.persist
(MergingActiveWindowSet.java:88)
at
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.ReduceFnRunner.persist
(ReduceFnRunner.java:380)
at
org.apache.beam.runners.dataflow.worker.StreamingGroupAlsoByWindowViaWindowSetFn.processElement
(StreamingGroupAlsoByWindowViaWindowSetFn.java:96)
...
```
