Need you to clarify some information. It appears the 1099-B you received in 2023 relates back to the ISO that you exercised in 2021, is that true? If so, then you may have to amend your 2021 return. If not, and it relates to transactions that occurred in 2022, then you will report it on your 2022 tax return.
Also, in your post you mention an example of purchasing the shares at $5 and then purchasing them again at $10. Do you mean that your strike price for the option was $5 while the stock price was $10, thus the discount was $5? If yes, then because these were ISOs, and it appears that the ISOs were sold less than a year after the exercise date, then that discount should have been reported as ordinary income on your W-2. Any gain above the reported ordinary income would be short-term capital gain.
You mention that the cost basis was left blank. That fact is not uncommon as brokers don't always know cost basis and thus, will leave cost basis blank or they might report cost basis as zero. In either case, the basis needs to be adjusted to reflect the price you paid. But first, we need to know to whether your 1099-B relates back to tax year 2022. Or whether it relates to tax year 2023.
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