In reply to keith-ratcliffe:
I knew I could count on the UKC community to help clarify my thoughts. I was surprised at how many people went for the honesty approach - I thought more would come from the 'Its their mistake' angle.
Anyway, composing the thread made me more certain of my original plan to come clean and tell them. The subsequent responses reinforced this approach.
So I rang them and asked to speak to someone who could sanction a deal - my ask was that I keep the second item (A lifejacket) if I could get it at a knock down price. The person I spoke to immediately thanked me for my honesty and said she would talk to a manager. After about a minutes tinkly music she came back and said I could have it at half price (Which was well below the discount price for a pair). I struck the deal, paid and she again thanked me very genuinely for telling them about their mistake. I felt good and ended up a satisfied customer.
A couple of points worth making about this incident.
1. We did intend to buy a second lifejacket but wanted to spread the cost. If it had been something we didn't actually want two off I may have felt differently.
2. This is not a big company I am dealing with - had it been an Amazon who recently wrote off £1000's of stuff sent to the wrong address telling the guy he could keep it - I may not have taken the honesty route. Plus I doubt I would end up speaking to a human if I tried to get in touch.
So thanks UKC for acting as the sounding board - I am reassured by your collective honesty.