Would these 2 invoices sent from the same limited company be classed as the same invoice
(my) Company name: XYZ
Invoice number: 185
date sent: middle of july
in the footer was XYZ trading as ABC
and then
(My) Company Name: ABC
date sent: end of October
Invoice Number: 185
client is saying that since invoice number 185 to XYZ trading as ABC was paid that makes invoice number 185 from ABC null and void, as they have proof of paying the invoice number 185
If you got one set of goods that you need to pay for, and you get two invoices, then you pay one invoice. Whether the two invoices have the same number or not doesn't matter.
If you got two sets of goods that you need to pay for, then you pay twice. If the sending company makes a mistake sending you two invoices with the same invoice number, you may refuse to pay the invoice, in which case you will have to pay the next, corrected, invoice.
In the UK and possibly other places, invoice numbers must be sequential. You can start at any number when you start the business (your first invoice can be number 1 or number 3,867 if you like), but further invoices must be sequential. If you send invoices 3,867 and 3,869 and there is no invoice 3,868 then you will have a lot of explaining to do to the tax office.
Probably not.
In any case, it wouldn't really matter unless they could show that a reasonable and objective person in their position would believe that the payment they made for goods/services rendered in July were also in payment for the services rendered in October - the crux of the contract is that you agreed to render goods/services in exchange for payment on both occasions.
You can adduce additional evidence to support this, for example any intervening payments made.
Since you have stated both trading names ABC and XYZ are for the same limited company, this is a single legal entity and many companies will setup one supplier account for one legal entity as this reduces their workload and associated costs with regards to credit control, due diligence compliance checks, and other administration (such as setting up a supplier account and getting it authorised etc) - they may even have a company policy which mandates this.
If they are using well designed accounts software then for a single supplier account then, it should not be possible for them to enter 2 different purchase invoices with the same invoice number/reference.
Solution: Your invoice numbers should be unique to the legal trading entity (in this case a limited company), not just the trading name (such as ZYC).
