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Run the following steps in the mongosh:
use test-db
db.createCollection( 'users' )
db.users.createIndex({ 'username' : 1 }, { unique: 1 })
db.users.insert({ "username" : "LDAP" })
db.users.insert({ "username" : "ldap" })
var bulk = db.users.initializeUnorderedBulkOp()
bulk.find({ "username" : "ldap" }).collation({ "locale" : "en" , "strength" : 2 }).upsert().updateOne({ "$set" :{ "username" : "ldap" }})
bulk.execute()