FULL PRODUCT VERSION :
jdk1.8.0_161.jdk

A DESCRIPTION OF THE PROBLEM :
toMap with an empty merge calls Map.merge using the throwingMerger function object as the remappingFunction for value fields. This causes a IllegalStateException on a duplicate key field that is actually the value field.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a list of n Strings where each String is the same value.
Convert the list into a stream.
Collect the list using the toMap function and supply no merger function to it.

List<String> list = Arrays.asList("a", "a", "a");
list.stream().collect(UUID.randomUUID(), String::toString)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
{UUID1 -> "a", UUID2 -> "a", UUID3 -> "a"}
ACTUAL -
java.lang.IllegalStateException: Duplicate key

ERROR MESSAGES/STACK TRACES THAT OCCUR :
t] Caused by: java.lang.IllegalStateException: Duplicate key a
    [junit] at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
    [junit] at java.util.HashMap.merge(HashMap.java:1254)
    [junit] at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
    [junit] at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    [junit] at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
    [junit] at java.util.HashMap$ValueSpliterator.forEachRemaining(HashMap.java:1625)
    [junit] at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    [junit] at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    [junit] at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    [junit] at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    [junit] at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
use a simple merge function (a, b) -> a