Learn English – How should I greet two people sharing the same first name in an email

Some sensitivity to age and formality is needed to answer this question. A formal note does not change in structure because it's being sent via email. There's nothing special or magical about email that gives one permission to be forward, rude, or insulting.

When writing to older persons, persons in authority, superiors, et al, I recommend a salutation and a complimentary close. These are not "wastes of time" by any means - they serve very specific functions if you are skilled in their use. Both the opening and the close allow you to frame your relationship with the recipient. For example:

Melissa:

Hi Melissa:

Dear Melissa:

These all have a different feel and express a different kind of tone. Paired with the proper close, you have no need for silly emoticons and winkies and such.

However, younger people will find these things to be quite strange and confusing. In sending email to anyone 25 and younger, I'd recommend being extremely curt and you might even be pushing the envelope by using punctuation.

Yes, though I think you're over-analysing it. By saying "have a good night", you're wishing them well for the entire remainder of the day, which is more informal than simply saying "see you tomorrow". An alternative would be "have a good evening".