validation - When should I prompt the user for duplicates entries?

We found a peculiar problem in our users when they enter their data. They tend to enter similar entries sometimes.

I'll cite an example here.

If there is a customer with the name ABC Electric Corporation already in the system, they sometimes enter the ABC Electric Corp. or sometimes ABC Elec. Corporation

We use this node package to show a set of similar names to the user when he is creating a new customer.

The question is when should we really prompt the user that he may be entering a duplicate customer?

We show the set of similar names when more than x% matches? What should be value of x?

Answer

I would personally go with a dropdown select that would show values already present in the database, starting at 2 or 3 chars entered.

That way, when your client enter ABC, he will see a dropdown with "ABC Electric Corporation" if already present.

Now, in case your user start by entering "BAC El..", you may try to use the NodeJS module you mentionned to find similarities, but it's not guarantee that with few chars, it will be efficient.

You can leave it up to your users (they can not be perfect ;)).

An other solution would be to suggest existing name AFTER they have submitted the form but this would require one more step for the user that may not be necessary (he may have this list even when entering a new, correct, name, adding one more step to something not necessary).