A DESCRIPTION OF THE PROBLEM :
Related issue : https://bugs.openjdk.java.net/browse/JDK-8042355 

A similar report was raised in the issue linked above but it still seems to be happening when a call to concat() is introduced in the pipeline. 


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
'peek' is called only once, i.e, output should be,

PEEK: a
result a

ACTUAL -
'peek' is called for all elements. i'e, output is,

PEEK: a
PEEK: ab
PEEK: abc
PEEK: abcd
result a


---------- BEGIN SOURCE ----------
public class Blah
{
	public static void main(String[] args)
	{
		Stream<String> stream = Stream.of("a", "ab", "abc", "abcd")
//                .sorted() // uncommenting this cause 'peek' to be called on all elements 
				.peek(s -> System.out.println("PEEK: " + s));

//        Stream<String> finalStream = stream;
		Stream<String> finalStream = Stream.concat(stream, Stream.of());

		String result = finalStream.findFirst().orElseThrow();

		System.out.println("result " + result);
	}
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
collect the sorted result into a list and start a new pipeline