Enthuware

So basically if you wanted Collectors.toMap to ignore duplicate key entries you'd pass in something like (a,b) -> a for the BinaryOperator argument.

as both collect and forEach are terminal operations, doesn't the code result in compilation error?

The code is not calling two terminal operations on the same stream. The call to collect returns a Map and forEach is called on that Map.
You might want to try compiling and running the code.