That isn’t an example of double dipping.
The military gets about 50% of their base wage if they stay in 20 years. They get 75% if they stay 30 years, etc. So, if someone does 20 years in the military and gets half their base wage, that’s the deal. If they then do another 20 years for the postal service to get another 50%, that’s still the deal, still the same rate, and it took 40 years to get 100% of a low wage.
Frankly, federal retirement pay isn’t that great. A person that spends 40 years saving in a retirement account on the civilian side can earn millions as a result, not $14-$80k or so per year the average single and double pension earner makes. Not to mention the civilian lives a far better life in most cases from the very beginning.
If you're referring to my comment about my friend and my BIL, you're right, it's not double dipping. What they have successfully done, which is not available in the private sector, is buy back their non vested years and apply it to their current tenure come retirement time.
I retired from the Air Force in 1988, as a Master Sgt. My retirement pay was $761 a month. After COLA increases, it is now $1,400 a month. It makes a dent in the house payment, but does not cover it all, but it is better than nothing.