No, it is not double. The 2017 Tax Cuts and Jobs Act reduced the marriage penalty quite a bit due to broadening the tax brackets. It is still there, especially for very high earners, but not noticeable for the vast majority.
Surely $600K as a couple income is even on the top endz even in tech. So, for most normal couples like where one earns $300K and the other $100K, filing as a married couple is better, no?
I think I misunderstood you, and did not realize tax slab meant tax bracket, as assumed you meant the US doubles the tax liability if you are filling married joint.
[assuming United States for everything here, in case it wasn't obvious]
In addition to the brackets, there's tons of other deductions, exemptions, etc that are higher or double for married-filing-jointly tax returns.
Just one example is the capital gains on selling your home: if you're filing singly you get an exemption on the first $250k of capital gains, but if you're married and filing jointly that exemption is on the first $500k of gains.
The tax brackets mostly eliminate the "marriage penalty", though there are some true edge cases that might make filing separately better. Generally speaking, most married couples do the same or better by filing together.
