√ Is it possible to create unique call-out numbers for references and repeat them in text in Word?
Some journals require references to be cited only once in the endnotes with unique number and be called out as many times as necessary in the body through superscripts. Because Word cannot handle non-consecutive footnote/endnote numbers, I don't think this schema is possible with Zotero (or MLZ) + Word plug-in, is it? If not, it'd be nice if the Zotero Word plug-in figures out a way to do this in the future, as this is a reference style that is not infrequently used.
the number is the citation (which can appear as superscript or - more commonly - in parentheses or square brackets), what you're calling "Endnotes" is actually a (numbered) bibliography. You can try this with a style such as Vancouver or IEEE.
Any idea to overcome this?
Frankly, having looked at Health Affairs, I'm not sure this can be done reasonably with Zotero. You can work around issues, but it's always going to be very messy dealing with an odd mix like that. I'm not terribly optimistic about adding support for that.
To be clear - this would be in Zotero, not in the Word add-in. The icon is between the magic wand and the paperclip.