Is this common for 5 months old Dual boiler? Should I worry? Always keep my machine clean, regular backflush. Customers service said this is considered cosmetic issue and even if they will replace it this can occur again.
How normal is that?

The BDB crew around here has seen this, either on their own machines or in posts from other people like you who have posted. It seems like a pretty widespread issue. I would actually be curious if anyone has a machine that does not have this issue.

Is it a problem? I don't know but I don't think so. The black coating seems to be some kind of slippery (non-stick) coating that helps keep the brew head clean. I have had my machine for several years, mine has had this problem for several years and it is still going strong. I noticed the bubbling early in the life of my machine, it has not gotten worse. I backflush my machine once a week.

Is it dangerous? I don't know and I hope not. I don't know what the coating is. I don't see how any of the flakes could get through the coffee cake and make it thorough the holes of the filter basket. I am not a chemist though, maybe there is a chemical mechanism.

(BTW, though I am not a chemist, I'll offer this. The mechanism for this peeling is theorized to be galvanic corrosion. The brew-head (metal under the coating) is aluminum. The screw that holds the screen up and threads into the brew-head is stainless steel. This makes some chemistry stuff happen that changes the surface of the aluminum brew-head and affects the bond with the coating, causing it to flake and bubble.)

Yes, it is common, and it appears to occur early in the lifecycle. Good news, at least for my BDB, it hasn't gotten worse and my machine is almost seven years old. Looks just like your pic.

Breville customer service replied that all consumer products are food safe and compliant with all applicable product safety regulations and requirements. I still think that's not normal for a 5 months machine that coat $1600 and not sure if that's going to affect coffee taste somehow.

yours doesn't look too bad. At least the coating doesn't peel off. Thanks for sharing

I've just noticed this on my own machine as well. Contacted Breville, and they say it's cosmetic and they won't fix it, even if I pay their service fee... I take it the consesus here is that this isn't a problem? (Even in a case this extreme?)

YES. they told me as well that it's cosmetic and absolutely normal which I doubt. My question is it Normal as it happens frequently оr normal as intended? To me this is more like bad customer service, poor construction and material and it's not suppose to happen for $1600 espresso machine. At least I was told by them that Breville products are food safe and compliant with all applicable product safety regulations and requirement so hopefuly that coating it's not toxic.

how old is your DB?

I would say "it's normal, as in Breville knows that once their machines get put to use for a while, the coating that makes them pretty on the showroom display will eventually get unattractive". It's been happening for years, people keep buying Breville machines, so it will continue to be normal for a long time. If the machine works, there's no need to fix it, yet. I wouldn't think it's worth replacing the grouphead unless you enjoy the labour of working on the machine and think the resale value will increase by more than the cost of the part.