also: みながみな
expression, adverb
every single one; each and every person
Emphasizes that the statement applies to all members of a group without exception. Often used in negative or critical contexts, such as みんながみんなそうじゃない (not everyone is like that).
みんながみんな賛成するわけではない。
Not every single person agrees.
みんながみんな同じ意見じゃないよ。
Not everybody has the same opinion, you know.
みんな simply means 'everyone' or 'all', while みんながみんな emphasizes each individual within the group, often in a limiting or negative statement.
全員 is a straightforward noun meaning 'all members', without the emphatic, distributive nuance of みんながみんな.
Reduplication of みんな (everyone) with the particle が, literally 'everyone, everyone', used for emphasis.