First off let’s be clear on what’s taxed and when

1) you buy stocks at a discount. You pay income tax on the discount. Let’s say you want $5k of stocks and so you pay $4500 for them with your discount. You pay income tax on $500. Not the whole amount. No way around this

2) you hold stocks for a while, long enough to meet the holding requirements. . Let’s say a year and it’s now worth $5500. You have a long term capital gain of $500. You pay tax on the cap gain. No way around that

Ultimately you didn’t get double taxed. You paid tax on two different types of gain. This is the same as if you took you leave paycheck, invested it in the market, sold for a gain. No different.