How to reproduce. 1. Deploy a simple configuration. For further reference: > db.version() 6.0.15 $ mongosh --version 2.3.0 Tested the same behavior on 7.0.12   2. Create some dummy structure: replset [direct: primary] admin> use foo switched to db foo replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).insertOne({ "name" : "Joe" , "age" : 98}) {   acknowledged: true ,   insertedId: ObjectId( '676358306ae75f8fcc95c3ab' ) } replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).find() [ { _id: ObjectId( '676358306ae75f8fcc95c3ab' ), name: 'Joe' , age: 98 } ] replset [direct: primary] foo> db.dummy.getIndexes() [ { v: 2, key: { _id: 1 }, name: '_id_' } ]   3. Let's create a new index for age: db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1}); age_1   4. So far, so good; Index created: replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).getIndexes() [   { v: 2, key: { _id: 1 }, name: '_id_' },   { v: 2, key: { age: 1 }, name: 'age_1' } ] 5. Now, let's create a new one on age but passing a collation parameter. But before that, let's increase the log and then create the index: replset [direct: primary] foo> db.setLogLevel(5) --- logs --- { "t" :{ "$date" : "2024-12-18T20:32:06.749-03:00" }, "s" : "D2" , "c" : "COMMAND" ,   "id" :21965,   "ctx" : "conn21" , "msg" : "About to run the command" , "attr" :{ "db" : "foo" , "client" : "127.0.0.1:44266" , "commandArgs" :{ "createIndexes" : "dummy" , "indexes" :[{ "collation" :{ "locale" : "simple" , "strength" :1}, "name" : "age_1" , "key" :{ "age" :1}}], "lsid" :{ "id" :{ "$uuid" : "03196d93-9a7e-4971-81c4-3e0314801e15" }}, "$clusterTime" :{ "clusterTime" :{ "$timestamp" :{ "t" :1734564702, "i" :1}}, "signature" :{ "hash" :{ "$binary" :{"ba se64 ":" rfdHUyasHlu2GLtKomzt7Zq3hRQ= "," subType ":" 0 "}}," keyId ":7392671011805593608}}," $readPreference ":{" mode ":" primaryPreferred "}," $db ":" foo"}}} { "t" :{ "$date" : "2024-12-18T20:32:06.749-03:00" }, "s" : "D2" , "c" : "COMMAND" ,   "id" :21955,   "ctx" : "conn21" , "msg" : "Applying default readConcern on command" , "attr" :{ "readConcernDefault" :{ "readConcern" :{ "level" : "local" }}, "command" : "createIndexes" }} { "t" :{ "$date" : "2024-12-18T20:32:06.749-03:00" }, "s" : "D2" , "c" : "REPL" ,     "id" :22548,   "ctx" : "conn21" , "msg" : "Applying default writeConcern on {cmdObj_firstElementFieldName} of {wcDefault}" , "attr" :{ "cmdObj_firstElementFieldName" : "createIndexes" , "wcDefault" :{ "w" : "majority" , "wtimeout" :0}}} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D2" , "c" : "REPL" ,     "id" :22549,   "ctx" : "conn21" , "msg" : "Waiting for write concern. OpTime: {replOpTime}, write concern: {writeConcern}" , "attr" :{ "replOpTime" :{ "ts" :{ "$timestamp" :{ "t" :1734564722, "i" :1}}, "t" :20}, "writeConcern" :{ "w" : "majority" , "wtimeout" :0, "provenance" : "implicitDefault" }}} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D3" , "c" : "STORAGE" ,   "id" :22414,   "ctx" : "conn21" , "msg" : "WT begin_transaction" , "attr" :{ "snapshotId" :141782, "readSource" : "kNoTimestamp" }} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D3" , "c" : "STORAGE" ,   "id" :22413,   "ctx" : "conn21" , "msg" : "WT rollback_transaction" , "attr" :{ "snapshotId" :141782}} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D4" , "c" : "STORAGE" ,   "id" :22419,   "ctx" : "JournalFlusher" , "msg" : "flushed journal" } { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "I" ,   "c" : "COMMAND" ,   "id" :51803,   "ctx" : "conn21" , "msg" : "Slow query" , "attr" :{ "type" : "command" , "ns" : "foo.$cmd" , "appName" : "mongosh 2.3.0" , "command" :{ "createIndexes" : "dummy" , "indexes" :[{ "collation" :{ "locale" : "simple" , "strength" :1}, "name" : "age_1" , "key" :{ "age" :1}}], "lsid" :{ "id" :{ "$uuid" : "03196d93-9a7e-4971-81c4-3e0314801e15" }}, "$clusterTime" :{ "clusterTime" :{ "$timestamp" :{ "t" :1734564702, "i" :1}}, "signature" :{ "hash" :{ "$binary" :{ "base64" : "rfdHUyasHlu2GLtKomzt7Zq3hRQ=" , "subType" : "0" }}, "keyId" :7392671011805593608}}, "$readPreference" :{ "mode" : "primaryPreferred" }, "$db" : "foo" }, "numYields" :0, "reslen" :242, "locks" :{ "ParallelBatchWriterMode" :{ "acquireCount" :{ "r" :4}}, "FeatureCompatibilityVersion" :{ "acquireCount" :{ "r" :6}}, "ReplicationStateTransition" :{ "acquireCount" :{ "w" :6}}, "Global" :{ "acquireCount" :{ "r" :6}}, "Database" :{ "acquireCount" :{ "r" :4}}, "Collection" :{ "acquireCount" :{ "r" :4}}, "Mutex" :{ "acquireCount" :{"r ":4}}}," readConcern ":{" level ":" local "," provenance ":" implicitDefault "}," writeConcern ":{" w ":" majority "," wtimeout ":0," provenance ":" implicitDefault "}," storage ":{}," remote ":" 127.0.0.1:44266 "," protocol ":" op_msg "," durationMillis":0}} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D2" , "c" : "QUERY" ,     "id" :22783,   "ctx" : "conn21" , "msg" : "Received interrupt request for unknown op" , "attr" :{ "opId" :64944, "knownOps" :[]}} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D3" , "c" : "-" ,         "id" :5127803, "ctx" : "conn21" , "msg" : "Released the Client" , "attr" :{ "client" : "conn21" }} { "t" :{ "$date" : "2024-12-18T20:32:06.750-03:00" }, "s" : "D3" , "c" : "-" ,         "id" :5127801, "ctx" : "conn21" , "msg" : "Setting the Client" , "attr" :{ "client" : "conn21" }} replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1},{collation:{ "locale" : "simple" , "strength" :1}}); age_1 From both ends, it seems it created that new index. But if you have paid attention you saw the same index name: age_1   Then checking, no new index was created: replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).getIndexes() [   { v: 2, key: { _id: 1 }, name: '_id_' },   { v: 2, key: { age: 1 }, name: 'age_1' } ] 6 . Now, let's try to use a similar command but passing an index name this time: replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1},{collation:{ "locale" : "simple" , "strength" :1},name: "some_name" }); MongoServerError[IndexOptionsConflict]: Index already exists with a different name: age_1 In that method, we see Mongo erroring out due to conflict.   Proposed fix. Mongo should handle both scenarios equally. Creating an index without a name on a field that already holds a similar index structure should trigger the same IndexOptionsConflict color: Color value is invalid message .   In the current condition, without an error conflict, the user can have a wrong understanding that the index was created because no error happened. replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1},{name: "some_name" }); MongoServerError[IndexOptionsConflict]: Index already exists with a different name: age_1 replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1}); age_1 replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1}); age_1 replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1}); age_1 replset [direct: primary] foo> db.getSiblingDB( 'foo' ).getCollection( 'dummy' ).createIndex({age: 1}); age_1 Both scenarios should be treated equally