a tricky billing question

looking at my bank statements and cheque stubs that go back almost 2 years I have noticed that I have paid a very large identical (there are no cents on the amount which would have made it 100% obvious) sum within 2 months of each other.

I can see this is a big coincidence and I can quite readily accept that it is something I could have done by mistake (ie paid the same bill twice)
It is also possible that the same amount reoccurred twice in a row in such a short space of time.

I have gone through the subsequent statements and cannot see that no refund was paid into my account (not sure if that could have been done like that anyway) .
I am also trying to hunt down the invoices and oil statements for that period but that is a bit of an upheaval and I could have thrown them out.
So if I suspect that the oil company has pocketed the 2 bills (or simply acepted the payment in error) but cannot be 100% or 99% certain what would be my best course of action?
Should I ask them for copies of the bills/statements for that period without mentioning the particular payments I am worried about?
Or just explain the situation and rely on their honesty/ helpfullness?

Do you know what company the two payments went to?

if you paid by cheque you can ask the bank for a copy of the cheque, albeit they may charge you

I don't think there would be a dispute that the 2 cheques went through as I have it in my bank statements.
But the oil company might say or believe (possibly correctly) that they were just 2 consecutive payments that came to the same amount.
I really need to satisfy myself of the truth of the matter and not make any false accusations .
But if I can't find the old oil bills then I suppose I have to approach the company itself so I wonder is there a way of doing this diplomatically and effectively (unless I can dig out the old oil bills)?

A few years ago, I was billed twice for oil within a few weeks of each other. The oil company had my credit card details and I think charged me for somone else's oil! Needless to say I asked them to remove my details from their system.