Mongo Giving 'duplicate key error' on non-unique fields

Mongoose doesn't remove existing indexes so you'll need to explicitly drop the index to get rid of it. In the shell:

> db.items.dropIndex('assets.serial_1')

This will happen if you initially define that field unique: true but then later remove that from the schema definition or change it to unique: false.

If you're using MongoAtlas, you can go to the collection -> click 'indexes' -> on the index you want to delete, click 'drop index'