what does gifting different people a second time do?

[a person]

im talking about people like skelly and stuff

[UNBREAKABLE]

Many of the characters have their own story arcs that only get revealed as you gift them nectar and ambrosia. There are some very useful rewards for fully bonding with certain characters too - skelly is one of them (and probably the first one you should fully bond with) but I won't spoil further.

For priority sake I would save the Olympian gods for last since fully bonding with them only really becomes relevant at the end of the story.

[chris.ferrantegerard]

As mentioned above, gifting additional Nectar increases the NPCs affinity. Increase enough affinity and you will gain access to a personal quest from the character and after doing that you can gift Ambrosia to reach full affinity.

Personally I'd suggest gifting everybody one Nectar first to unlock everybody's keepsakes first and then work on upgrading NPCs affinity. Once you buy the Regional Keepsake Collection you can swap keepsakes after a boss fight letting you use some keepsakes as a 'starter' keepsake then swapping to another keepsake later for the tougher stages and bosses (ie: Elysium and up).