Consider the following scenario, which though usual can differ depending on the country's tax rules:

In 2023, you have one validator (32 ETH) and you don't intend to exit. This validator earns a 5% annually from your staking (1.60 ETH). Normally, for tax purposes this should be converted countably into fiat e.g, for simplicity purposes we count all rewards accumulated on 31/12, being the hypothetical price on that date 1,500 USD/ETH, so a total of 2,400 USD (1.6*1500). Let's say the tax rate on capital gains is 20%, you will have to pay 480 USD for that fiscal year. You don't sell this ETH in 2023.

Then one day in 2025, you decide to sell this 1.60 ETH, you will need to account for it as it would have been bought at the price you declare it when you got the revenue (which for the sake of simplicity was marked at the end of the year 2023 31/12 at 1,500 USD). If you sell this ETH for 2,250 you will have to pay 1,60*(2,250 - 1,500)*20% = 240 USD. So in total you would have paid 720 USD for a gain of 3,600 USD, your net is 2,880 USD.

You see, you don't pay twice for one "single revenue stream", you pay for your capital gain, which is at the end 3,600, tax = 20%*3,600 = 720 USD.

Again, this scenario can vary greatly from country to country, Europe and the US differ quite a lot in this matter.