I believe Toastie is entitled to a replacement of the correct type or, if that is not possible, a refund. Based on my experience of e-shop there are two possibilities:
1 e-shop have a dual SIM version but they sent the single SIM version in error, in which case they will replace it with the dual SIM version.
2 They don't have a dual SIM version in which case they will refund him the price.
Nobody is suggesting that it isn't e-shop's fault or that they are not responsible for correcting their error. I have never know e-shop to refuse to correct an error. My gut feeling is that it might be a bit more confused than that. If you search mobiles on e-shop and filter for only Sony models it displays 3 model Z5s, black, green and white. They are all described in words as dual SIM. If you add a filter for dual SIM only it only displays two model Z5s, black and green. It is possible, but not certain, that the black and green models are really dual SIM and the white one isn't. In that case, if it was the white one that he ordered, they might be able to offer him a an alternative colour. It's also possible that they are all single SIM.
I raised the full model number as a general information issue. All smart phones and tablets come in a variety of generic model numbers, like Sony Experia Z5. Within that generic model number there are often a number of variants such as single/dual SIM, 16/32/64 GB storage, 2/3/4/6/8 GB memory. You can only tell which variant you are getting by looking at the full model number. Toastie says his was E6653 but he doesn't say where he got that information from. As far as I can see it isn't on any of e-shops web pages describing the different models. He may have got it from the box when it arrived, in which case it was too late. It might, however, been on the email that e-shop send to confirm the order. In that case he could have contacted them immediately and stopped the order before it was delivered.