Idea needed: how to prevent user from signing more than once

I have a 5-step signup page.
User enters business info, staff information, office info, doctor information. Clicks save. Then page goes to payapl or whatever payment method and pays and now they're registered for the site.

Suppose business name is "abc company". How can I make sure this company doesnt sign up more than once? for example, office ABC Company has signed up...then they make a mistake and signup again and pay again. How can I prevent that? I thought about company name, address or office info but I dont that's full proof...for example...what if on the first signup...user enters "ABC Company" and sign up but on second signup they enter "ABC Comp"...

should it be by someone's email? address, a combination? not sure how to prevent an office from signing up over and over again?
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Dale BurrellDirectorCommented:
Unless you require some unique, verifiable information there is nothing you can do.

So for example most countries issue a company registration number for companies that are registered. So if you only deal with registered companies you could use that number. You may still need to look up the numbers given to ensure they are valid, but maybe at this point you just trust them?

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CamilliaAuthor Commented:
>>company registration number for companies that are registered

So how does this work?
1. Company signs up
2. Company gets a registration number
3. How is this registeration number used if that same company tries to register again? Ask for their registeration number during signup? Not sure how the registeration number would help in preventing a company from signing up again and again...

Yes, you'd think a company wouldnt sign up more than once but you never know!
Dale BurrellDirectorCommented:
"..most countries issue a company registration number.." e.g.

I don't think there is any way can stop a company registering again. My suggestion is to use some "official" information which you can verify and then remove them again if they have tried to trick you.
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CamilliaAuthor Commented:
I looked at that site...not sure how they're using registeration number....

>>"official" information which you can verify
That's what i dont understand. Company signs up. I issue them a number. How this number can prevent them from signing up again? How is this number used for verification? They have username/pwd to sign in with..

During signup, they have to enter a phone number, website or email. Was thinking about using their email "domain" as verification that they've already signed but this is not good. Phone number...maybe?
Dale BurrellDirectorCommented:
That site ISSUES registration numbers for registered companies in the UK - its an OFFICIAL source.

None of the details you mention are going to prevent them from signing up again. I don't think there is a way, you can for sure check that the email address and phone number are unique, but that won't stop anyone from changing them.
CamilliaAuthor Commented:
Yeah, i dont think so either but my manager keeps asking for a solution. I will break the news to him.
The point is you have to ask them for something that is definitely unique and must be entered correctly. Like in the US you might ask for a SSN for a person to make sure its unique even then its problematic. For a business you might ask for their Tax Identification Number.

No matter how you try to make sure they enter information (address, email, etc) there is nothing stopping some 2nd person in the company from coming along (accidentally) and signing up again using a slight variation on the information. Even if you asked for a SSN or TaxID they could typo it and you have no way of checking it against an official government database to make sure it matches that company.

You can check against what information you have to make sure no duplicate user, but unless you come up with a strong candidate for a "unique" entry you cannot be absolutely sure they didnt sign up twice. The more detailed you check the less mistakes will happen.

Also its not really your problem excepting to be conscientious. Its their money. If they sign up twice with different information how is that your fault? Happens all the time in big companies.

The party line would be "I'm sorry sir... No sir how was I to know this was the same company, the information wasn't the same entirely. You used a different email and a different name and a different variation of the address. We do extensive checks but it is ultimately the responsibility of the client not to sign up twice. I can cancel this duplicate account but I will not be able to give you a refund of the funds (or give them partial back)."

In short, you can do as much work as you like to try to account for user mistakes, but in the end you will have to have a policy to deal with what "leaks" through.
CamilliaAuthor Commented:
Thank you both. Makes sense.
Think also about this...

Lets say you check against a company name or an address and determine they already have an account. Now what do you do?

If you inform the user and they arent actually the same company then you have just given out potentially personal or confidential information about one of your clients. If you're form does it's duplicate checking before the "paypal" portion (to make sure they dont pay twice) then your form is basically a "snif" capable window into your client list.

Say I want to know if my competition is using your services. I just plug in their information and try it out... now I know.

If you're paying attention when you sign up for stuff you may be able to get some insights into the thinking a company put into this type of situation.

It may not be something you care about or may not be relevant to you, but be careful what you're doing and how you report what is happening.

This gives you even more argument for duplicate account policies and why you let them happen to an extent.
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