This problem can occur in process models where there is a joining inclusive gateway and there are two nested scopes on a path leading to that gateway. An execution must be in a wait state within the inner scope and the inner scope must have no outgoing sequence flow.
Example diagrams attached. When Task A is completed first, the inclusive gateway immediately fires, leading to overall two instances of Task C.
Reason: poor implementation of org.camunda.bpm.engine.impl.bpmn.behavior.InclusiveGatewayActivityBehavior#isReachable