Create a document with array of objects, say "children", and those array element objects having a field, say "childId". Eg: { "children": [{ "childId": "child1" }, { "childId": "child2" }] } Create an unique index on children>childId Create another document with same data, but different childId values. Eg: { "children": [{ "childId": "child3" }, { "childId": "child4" }] } In Mongodb Atlas web, clone the document to create a new document with same details. You'll get an error: "Unexpected error" In Mongodb compass, object view ( {} ), try doing the same, will get issue most of the times, the screen goes blank! In Mongodb compass, list view, try doing the same, it works as expected most of the times. If cloning works, without any error, do the following: update the second document to add another object to "children" array with "childId": "child1" (to be a duplicate of childId of an element of first document). You'll get the error.