FULL PRODUCT VERSION :
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
I have tried to convert a stream into a map. Unfortunately there were duplicated values in keys. An exception was thrown, which is OK. The only problem is that this exception message points not to the duplicated key, but to the associated value.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Make a stream of some objects with two properties A and B.
2. Ensure that this stream contains two objects with the same value for A property.
3. Convert this stream to a map using A property as a key.
4. Check wrong exception message.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
java.lang.IllegalStateException: Duplicate key KEY1
ACTUAL -
java.lang.IllegalStateException: Duplicate key VALUE

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.IllegalStateException: Duplicate key VALUE

at java.util.stream.Collectors.lambda$throwingMerger$113(Collectors.java:133)
at java.util.stream.Collectors$$Lambda$3/1361960727.apply(Unknown Source)
at java.util.HashMap.merge(HashMap.java:1245)
at java.util.stream.Collectors.lambda$toMap$171(Collectors.java:1320)
at java.util.stream.Collectors$$Lambda$5/2085857771.accept(Unknown Source)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
    @Test
    public void test() {
        List<Pair<String, String>> list = new ArrayList<>();
        list.add(new Pair<>("KEY1", "VALUE"));
        list.add(new Pair<>("KEY1", "VALUE"));
        try {
            list.stream().collect(Collectors.toMap(Pair::getKey, Pair::getValue));
        } catch (IllegalStateException e) {
            assertThat(e.getMessage()).isEqualTo("Duplicate key KEY1");
        }
    }
---------- END SOURCE ----------