They just wouldn't let it lie, would they? Fat chance. These two are proving really hard to separate. Turn your back for just one moment and they're at it again. Every breaktime you'll find them out in the corner of the playground trying to knock seven shades of printer's ink out of each other. There seems to be an almost elemental force at work here.
And the thing is, we almost let ourselves believe that the heat was coming out of this one. This one being, of course, the Mail versus the Express.
When their relationship hit a new low a few months back, it looked as if the whole matter was heading for the courts but they pulled back from the brink and it seemed that both sides had finally been shamed into mending their ways. Almost miraculously, they reached a gentleman's (a term they managed to use without any hint of irony) agreement to refrain from calling each other really hurtful names such as "Nazi" and "pornographer".
The agreement was not to last. When it comes to burying the hatchet, neither side seems to have much of a grasp on what the expression actually means. This time (but who's really counting?) the Mail is the aggressor.
It sent a mail (no pun intended) shot to Express readers that attempted to convince them that their newspaper wasn't very good. Not only that, it also attempted to convince them that the Mail was quite a decent newspaper in comparison.
And so the Express called in the Advertising Standards Authority, which yet again got its chance to give a fair impersonation of the well-meaning warder Barraclough in that old prison sitcom Porridge. In short, it refused to take action, while agreeing that some of the Mail's claims were unfounded. But isn't it time to face up to the fact that all this bickering can actually be damaging to both titles?
Doesn't it make the mid-market a less attractive place to be for advertisers?
Shouldn't someone step in and start banging a few heads together?
Tim McCloskey, a managing partner of OMD UK, says it's not that simple: "I think most people in our business aspire to being legal, decent and honest. But I feel we have to question not the integrity or the desirability of the ASA but the fact that, like the Press Complaints Commission, it lacks legislative bite, so nobody is ever really taken to task. It is my own belief that newspapers do not go out to break any of these codes deliberately, but that to make their own messages and their products punchier than a rival's, they all cross the line on occasion, mostly without realising it."
But McCloskey doesn't think that this detracts from the genuine battle now unfolding in the mid-market? "For the first time in a generation the Express has three titles with increasing sales and there's a real battle between these two houses.
And the beneficiaries of all this activity will be the advertisers and the consumers. Both are getting more choice and I suspect better products.
Certainly it creates more interest in the national newspaper medium among clients and agencies. That's exactly what the newspaper business needs to take money from other media," he says.
On the other hand, this is an era in which the medium's traditional revenue base - both from a circulation and an advertising sales point of view - is coming under ever-greater threat. The medium itself is slowly waking up to this reality and it has at least recognised the need for a joint industry marketing body. But the newly created Newspaper Marketing Association has neither the means nor the inclination to intervene in this sort of bickering match.
The bottom line, though, will be whether it all makes advertisers entertain second thoughts about spending their money here.
Tom George, the deputy managing director of ZenithOptimedia, can't see that happening: "From an advertiser perspective, I'm not sure they really are concerned about the sniping between the Mail and the Express as long as the mid-market remains healthy in general. The Mail has been healthy for some time now and one way you could look at recent events is to say that it shows that (Express proprietor Richard) Desmond means business.
But the Mail is so strong currently that often it's mandated by the advertiser - that's a measure of the challenge Desmond faces. But at least he's stirring up the marketplace. If his determination results in a better product with increased circulation, posing a realistic challenge to the Mail, then they will be happy. So in that context, I'm not sure they worry too much about what happens in public. This, after all, is how press barons behave - and it has gone on since time immemorial."
George doesn't think any of this has been damaging to either the Mail or the Express as brands. Marc Mendoza, the chief executive of Media Planning, sort of agrees. He believes that some advertisers actually find all of this hugely entertaining: "It makes for a bit of interest in a dull category, doesn't it? It's a spark of life. It always makes for an interesting story when you have a would-be usurper taking on the establishment and the added ingredient here is the pomposity of Associated and the supposed darkness of Desmond. It makes for a fantastic story."
But that doesn't mean it's good for the sector. Or, to be more accurate, Mendoza adds, for the Express in particular. "Desmond gets a kick out of shaking things up but obviously not all that he wants comes to fruition.
There's no harm in being the new boy and seeking to rewrite the rules and there's no harm in having big ambitions, but some of it is wishful thinking. Associated is a formidable machine. I'm old enough to have seen several different management teams coming in and telling me they're about to turn around the Express. It hasn't happened yet," he says.
Paul Thomas, a managing partner of MindShare, says we should get used to the bickering - it's going to continue. A lot of it, he argues, is down to the chemistry of the individuals involved: "It kicked off when Associated wanted to buy the Express and had it nicked from under their noses for less than they had been offering. And Desmond keeps infuriating them by poking a stick at them now and again. Readers do get to hear about it - the side of the story given to them by whichever paper they favour - but I don't think it's something that bothers them. It's only in the industry that we see it all."
So - yet another storm in a tea cup? Actually, there could be more profound implications, Thomas says. "Dirty tricks have always been part and parcel of the newspaper business. This makes you wonder, though, whether there is any real role for the Newspaper Marketing Association given that everyone - and it's not just confined to the Mail versus the Express - spends so much time and money having a go at everyone else," he concludes.