ah I know this one

it's when you try to create multiple mongoDB instances

I'm guessing you have some sort of init function to initialise your db

what it returns (the mongoDB instance) needs to be defined out of the function and then the init function for MongoDB will only be run when that mongoDB insteance doesn't exist yet

You probably ahve something like this.

mongoose.connect(process.env.MONGODB_URI, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
  serverSelectionTimeoutMS: 5000,
  socketTimeoutMS: 45000,
});

const UserSchema = new mongoose.Schema({
  // My schema stuff
});

const User = mongoose.model("User", UserSchema);

You need to save User outside of the function returning it so you can reuse it

hope this helps