Sounds like you're either confusing turns with rounds OR you're using a house rule regarding a group of monsters acting on the same turn.
No I'm aware of how the round/turn structure works. I was just saying that in a one on one fight retaliation will not allow more than one attack per turn. You are correct that in the event that the 19th level Barbarian/Fighter is damaged by two separate creatures on two turns he will be able to retaliate twice that round. Which he had to gve up quite a lot to do. However wouldn't it be wiser for the hobgoblins to avoid melee attacking the Barbarian/Fighter at all? It would be best to use skirmishing tactics and ranged attacks to avoid it all together, and after the first retaliation intelligent monsters would likely change their tactics.
What you have here is a clear power-up over other characters that are possible in the PHB. This is objectively better than a Battle Master using Counterattack.
If you're operating from the premise that the PHB Fighter is "weaker" than other classes, then this may be seen as a feature to you. Similarly, if you want to encourage multi-classing into fighter, this may be seen as a feature to you. However, if the objective of your design is to make the fighter more interesting without a significant power boost, then this is something to be aware of.
Another example would be being able to use shield (via Magic Initiate feat or levels in a spellcasting class) and to make an opportunity attack in the same combat round.
You are correct that I am operating from the premise that the PHB Fighter is weaker than other classes. About half of the fighter's damage over the course of a day comes from the core class while the rest needs to come from the subclass.
For a 20th level Fighter the damage from action surge come to 129.6 damage over the course of an adventuring day.
For a 20th level Battle Master only using riposte (the most damaging maneuver) it comes to 172.5 damage over the course of an adventuring day. (or 136.5 if you use other maneuvers that give you the bonus damage)
For a 20th level Champion, Superior Critical with a great sword or maul is only dealing 16.8 damage over the course of the 24 attacks that the Battle Master had to make to use up an entire day's worth of resources. At most if you had advantage on every single attack you would reach 33.6 damage.
Now from this we can see that the Champion is severely under-powered compared to the Battle Master.
However if we compare the Fighter to the Paladin we can see that the Fighter/Battle Master is still under-powered.
If a 20th Paladin uses every spell slot to smite on every successful hit he will do 310.5 damage over 15 attacks.
Compare the daily damage of 302.1 damage of the fighter using riposte on every superiority die (which is pretty close) or 266.1 damage for using another maneuver that deals the superiority die in damage. For each maneuver you use that does not deal damage it reduces your damage even further.
We can then sadly compare the Champion fighters 163.2 damage when he has advantage on every attack.
This doesn't even include all of the other features the Paladin gets like lay on hands, auras, divine health, and the Oath abilities.
So yes I think the Fighter needs some love. He isn't even the best at fighting.
Math
All calculations made against a Hobgoblin with AC 18
Improved Divine Smite vs 2nd Extra Attack
11th level Paladin vs 11th level Fighter
Both with a +9 Attack Bonus
.45(2d6+5+1d8)+.45(2d6+5+1d8) = 14.85 vs 16.2 = .45(2d6+5)+.45(2d6)+5+.45(2d6+5)
Paladin Divine Smite damage per day
3 3 (2d8)*3 = 27 damage from 3 attacks
5 4 2 (2d8)*4+(3d8)*2 = 36+27 = 63 damage from 6 attacks
7 4 3 (2d8)*4+(3d8)*3 = 36+40.5 = 76.5 damage from 7 attacks
9 4 3 2 (2d8)*4+(3d8)*3+(4d8)*2 = 36+40.5+68 = 144.5 damage from 9 attacks
11 4 3 3 (3d8)*4+(4d8)*3+(5d8)*3 = 54+54+67.5 = 175.5 damage from 10 attacks
13 4 3 3 1 (3d8)*4+(4d8)*3+(5d8)*3+(6d8) = 54+54+67.5+27 = 202.5 damage from 11 attacks
15 4 3 3 2 (3d8)*4+(4d8)*3+(5d8)*3+(6d8)*2 = 54+54+67.5+54 = 229.5 damage from 12 attacks
17 4 3 3 3 1 (3d8)*4+(4d8)*3+(5d8)*3+(6d8)*3+(6d8) = 54+54+67.5+81+27 = 283.5 damage from 14 attacks
19 4 3 3 3 2 (3d8)*4+(4d8)*3+(5d8)*3+(6d8)*3+(6d8)*2 = 54+54+67.5+81+54 = 310.5 damage from 15 attacks
20 4 3 3 3 2 (3d8)*4+(4d8)*3+(5d8)*3+(6d8)*3+(6d8)*2 = 54+54+67.5+81+54 = 310.5 damage from 15 attacks
Action Surge Damage over a 3 short rest day
3 .45(2d6+3)*3 = 13.5 damage from 3 Attacks over 3 rounds
5 .45(2d6+4)*6 = 29.7 damage from 6 Attacks over 3 rounds
7 .45(2d6+5)*6 = 32.4 damage from 6 Attacks over 3 rounds
9 .45(2d6+5)*6 = 32.4 damage from 6 Attacks over 3 rounds
11 .45(2d6+5)*9 = 48.6 damage from 9 Attacks over 3 rounds
13 .45(2d6+5)*9 = 48.6 damage from 9 Attacks over 3 rounds
15 .45(2d6+5)*9 = 48.6 damage from 9 Attacks over 3 rounds
17 .45(2d6+5)*18 = 97.2 damage from 18 Attacks over 6 rounds
19 .45(2d6+5)*18 = 97.2 damage from 18 Attacks over 6 rounds
20 .45(2d6+5)*24 = 129.6 damage from 24 Attacks over 6 rounds
Battle Master Damage over a 3 short rest day (+ additional damage if using riposte)
3 (4d8)*3 = 54 damage +24
5 (4d8)*3 = 54 damage +24
7 (5d8)*3 = 67.5 damage +30
9 (5d8)*3 = 67.5 damage +30
11 (5d10)*3 = 82.5 damage +30
13 (5d10)*3 = 82.5 damage +30
15 (6d10)*3+3d10 = 115.5 damage +36
17 (6d10)*3+3d10 = 115.5 damage +36
19 (6d12)*3+3d12 = 136.5 damage +36
20 (6d12)*3+3d12 = 136.5 damage +36
Comparing Riposte vs any other damaging maneuver
.45(2d6+3+1d8) = 6.525 damage vs 4.5 damage difference of 2.025 at 3rd level
.45(2d6+5+1d12) = 8.325 damage vs 6.5 damage difference of 1.825 at 20th level
Champion extra damage from Superior Critical
.1(2d6) 0.7 damage per attack or .14 damage per attack with advantage