Is money earned from a project similar to another person's project, which was viewed by hacking into their email, considered unlawful (haram), even though the study and research for the project were done through independent efforts unrelated to the hacking?

What you have done by accessing others' emails and computers is forbidden (haram) because it involves spying and harming Muslims, and it may even lead to seizing their wealth. You must repent sincerely (tawbah nasuhah) by feeling remorse, abandoning the sin, and resolving not to return to it, while also redressing injustices or seeking forgiveness from those you wronged. Increase your acts of obedience. As for your project, there is no harm in it as long as its field is permissible.