Document already exists

Working with elastic 5.4.3 on 3 nodes cluster with 1 shard and 2 replicas.
During a test, node-3 was killed. At the same time, tried to index a document.
This scenario should result in creating the document successfully.
What happened was that elastic responded with

org.elasticsearch.index.engine.VersionConflictEngineException: [events][5523]: version conflict, document already exists (current version [1])

The id of this document was used only for this operation. I believe that the killing caused elastic transport client to try and index the document twice.
Could it be?
Eventually, the document was found in the index but still want to know what caused the duplicate error.
