If user provides non-unique uid for a stream, error message does not give a clue where does the error reside in client code. Including user-provided uid into error message should narrow the search area.
In 1.6.2 we see the message that was introduced in 1.2.0:
java.lang.IllegalArgumentException: Hash collision on user-specified ID. Most likely cause is a non-unique ID. Please check that all IDs specified via uid(String) are unique. at org.apache.flink.streaming.api.graph.StreamGraphHasherV2.generateNodeHash(StreamGraphHasherV2.java:180) at org.apache.flink.streaming.api.graph.StreamGraphHasherV2.traverseStreamGraphAndGenerateHashes(StreamGraphHasherV2.java:112) at org.apache.flink.streaming.api.graph.StreamingJobGraphGenerator.createJobGraph(StreamingJobGraphGenerator.java:134) at <rest of stack>.