Use addCallback rather than callback= in asyncfutures.all() (#6850)

* Use addCallback rather than callback= in asyncfutures.all()

Addresses part of #6849

* Stop using do notation for #6849

* Update example style

tests/async/tasyncall.nim

@@ -40,6 +40,16 @@ proc testVarargs(x, y, z: int): seq[int] =
 
   result = waitFor all(a, b, c)
 
+proc testWithDupes() =
+  var
+    tasks = newSeq[Future[void]](taskCount)
+    fut = futureWithoutValue()
+
+  for i in 0..<taskCount:
+    tasks[i] = fut
+
+  waitFor all(tasks)
+
 block:
     let
       startTime = cpuTime()
@@ -57,6 +67,13 @@ block:
 
     doAssert execTime * 1000 < taskCount * sleepDuration
 
+block:
+    let startTime = cpuTime()
+    testWithDupes()
+    let execTime = cpuTime() - startTime
+
+    doAssert execTime * 1000 < taskCount * sleepDuration
+
 block:
     let
       startTime = cpuTime()