If you already converted your IRA with after-tax contributions to Roth, then you have no remaining basis in your IRA. Now if you rollover your 401k with only pre-tax money and earnings, to an IRA, and convert that to Roth, it will be entirely taxable.
"If so, does it mean that my after tax money is being double taxed?" - can you clarify what after tax money you are referring to here since you previously converted it all to Roth already; are you planning on doing more non-deductible IRA contributions and converting those to Roth i.e. backdoor Roths? Note if you have no 'earned' income in retirement, then you can't make IRA contributions.
But you will not be "double taxed". Say you try to do backdoor Roth conversions while also having pre-tax IRA balance (and if you have multiple Traditional IRA accounts they are just considered as one), then the conversion is considered taken in pro rata manner from both your pre-tax and after-tax (basis). The portion of the conversion that is after-tax money is converted tax-free, and you pay tax on the pre-tax portion; your remaining after-tax money / basis is carried forward, and will be converted tax-free at a later date - but it won't fully convert until the IRA has a zero balance, so it's just a question of timing.
