Only show duplicate id warning if really from _id

Fixes #1980

packages/observe-sequence/observe_sequence.js

@@ -116,7 +116,8 @@ ObserveSequence = {
 
             var idString = idStringify(id);
             if (idsUsed[idString]) {
-              warn("duplicate id " + id + " in", seq);
+              if (typeof item === 'object' && '_id' in item)
+                warn("duplicate id " + id + " in", seq);
               id = Random.id();
             } else {
               idsUsed[idString] = true;

packages/observe-sequence/observe_sequence_tests.js

@@ -483,7 +483,7 @@ Tinytest.add('observe sequence - number arrays', function (test) {
     {removedAt: [{NOT: 1}, 1, 1]},
     {addedAt: [3, 3, 1, 2]},
     {addedAt: [{NOT: 3}, 3, 3, null]}
-  ], /*numExpectedWarnings = */2);
+  ]);
 });
 
 Tinytest.add('observe sequence - cursor to other cursor, same collection', function (test) {