(From edited Slack discussion)

There's a difference between the old and new shells.

collMod should return a CannotConvertIndexToUnique error with a list of conflicting documents.

Test collection:
db.apples.insertMany( [

{ type: "Delicious", quantity: 12 }

,

{ type: "Macintosh", quantity: 13 }

,

{ type: "Delicious", quantity: 13 }

,

{ type: "Fuji", quantity: 15 }

,

{ type: "Washington", quantity: 10 }

,
] )

test> db.runCommand( {
collMod: "apples",
index: {
keyPattern:

{ type: 1 }

,
unique: true
}
} )

mongosh response:
MongoServerError: Cannot convert the index to unique. Please resolve conflicting documents before running collMod again.

mongo shell response
{
"ok" : 0,
"errmsg" : "Cannot convert the index to unique. Please resolve conflicting documents before running collMod again.",
"code" : 359,
"codeName" : "CannotConvertIndexToUnique",
"violations" : [

{ "ids" : [ ObjectId("62a2015777e2d47c4da33146"), ObjectId("62a2015777e2d47c4da33148") ] }

]
}