It's a strange warning, it's the first time I've heard of it, how can it happen?
How can you or the person who wants to scam you make this invalid transaction?
when he uses a wallet support RBF, you will get RBF flag error.
How can you send and get this error?
With rbf enabled the network is designed to essentially ignore your last transaction and favour the one with the higher fee (default mempool consensus anyway). The number of inputs and outputs don't have to be the same.
I've done it in the past when making a mistake transferring funds somewhere I didn't want them to end up and used a version of electrum to sign a new transaction while offline (that didn't know of the other transaction) and broadcast it without a problem.