FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux workstation 4.4.0-66-generic #87~14.04.1-Ubuntu SMP Fri Mar 3 17:32:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
When using Collectors.toMap(keyMapper, valueMapper), a duplicate key is expected to throw an exception. The exception is thrown. However, the message incorrectly reports a duplicate key when it is reporting the existing value stored at a particular key.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
 Run something like this:

Arrays.stream(new Character[]{'a', 'b', 'a'})
  .collect(Collectors.toMap(Function.identity(), Character::toUpperCase));

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
An IllegalStateException with a sensible message.
ACTUAL -
The following exception message:
java.lang.IllegalStateException: Duplicate key A

Note that 'A' is a value in the map not the key, which would have been 'a'.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public class DuplicateKey{

  public static void main(String[] args) {
    Arrays.stream(new Character[]{'a', 'b', 'a'})
        .collect(Collectors.toMap(Function.identity(), Character::toUpperCase));
  }
}
---------- END SOURCE ----------