Setup: > use test switched to db test > db.test.drop() true > db.runCommand({ createIndexes: "test" , indexes : [ { key : { x : 1 }, name : "x_1" } ] }) { "createdCollectionAutomatically" : true , "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } Reproduction (note that name is different from above): > db.runCommand({ createIndexes: "test" , indexes : [ { key : { x : 1 }, name : "x_2" } ] }) { "createdCollectionAutomatically" : false , "numIndexesBefore" : 2, "numIndexesAfter" : 2, "note" : "all indexes already exist" , "ok" : 1 } > The problem: since the server returned { ok : 1 } that implies that an index with the name "x_2" must exist. But that is not true. Expected result: Some sort of error message indicating that the second index was NOT created. Perhaps similar to the error message when trying to create a second index with conflicting options: > db.runCommand({ createIndexes: "test" , indexes : [ { key : { x : 1 }, name : "x_1" , unique : true } ] }) { "ok" : 0, "errmsg" : "Index with name: x_1 already exists with different options" , "code" : 85, "codeName" : "IndexOptionsConflict" } > Suggested errmsg: Index with the same keys but a different name already exists.