Shared calendars on iPad

What’s the difference between “Shared BY …” and “Shared WITH …’ when trying to share calendars between my husband and myself?

Now I have duplicate calendars on both iPads. Calendar appointments/items display on my husband’s calendar, BUT each appointment/item displays TWICE on my calendar, which drives me crazy because it takes up way too much space on my monthly view.

I’ve tried to share using “shared WITH” and “shared BY” and either my or my husband’s calendar item disappeared.

I’m going insane trying to synchronize calendars!