Fanaelialae
Legend
It could be argued that way, but not convincingly.
Rules already exist for the same effect stacking/not stacking. The new duration replaces the old. This is why we can't claim to be immune to incoming long duration debuffs if we are on the last round of a current debuff.
The effect of using your bonus action to Rage while you are already Raging is that it lasts longer at the cost of another use of the Rage ability, and you don't get many. It makes sense (so, I'm Raging longer! What of it? I paid the price!), and can hardly be seen as an immoral act or a cheezy meta-game exploit.
The rules let you do what they say they let you do. It would take a line forbidding you to use a bonus action to Rage while already Raging to prevent it. It already has permission simply by saying what you can do and how you do it.
Sure, a DM could houserule a nerf, but why? He's not getting anything for free. He's probably losing out on some rounds of duration over the whole day. Let him do as the rules say; it makes sense in-world, he's paying the cost, nothing has gone wrong here that needs the nerfbat.
I disagree. It's a common sense reading, albeit a very rules-lawyery one.
"On your turn you can enter a rage as a bonus action."
You can't enter something if you are already in it. Say you are standing in your home. How do you re-enter it without leaving it first? You can't. Similarly, you can't enter a rage if you are already in a rage (I'm speaking of the emotion in this instance, not the mechanic). It wouldn't make logical sense.
As such, it has nothing to do with stacking of effects. It's merely how the activation clause is phrased.
That said, I don't think it's overpowered either. I've seen many barbarians at my table, and I'm not certain I've seen the duration come into play ever. I don't think it's that big a deal, and I don't think running out the clock is the way to go unless you find a way to first incapacitate the barbarian (ie, banishment).
That said, at 20th level the barbarian gains the ability for infinite rages. While I'm not one to say an ability is broken based on a capstone feature, there are probably some DMs who would object to the idea of a 20th level barbarian raging 16 hours a day without even so much as a break in between.
Rule It as you see fit. All I'm saying is that the rules can be read such that it isn't permitted to enter a rage while in a rage.