Business Insider counts at least six times that knowledgable people warned Trump transition and administration officials about Michael Flynn before the president finally forced the national security adviser to resign on February 13, 2017.
President Barack Obama was first to warn President-elect Trump himself on November 10, 2016:
"President Obama underscored with President-Elect Trump how important a role the NSA is — and how it demanded a serious person with sound judgment, impeccable credentials, and unimpeachable character," [a former Obama White House] official said.
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...Obama fired Flynn as head of the Defense Intelligence Agency (DIA) in 2014.
Not only did two separate media outlets report Flynn's lobbying work for foreign interests just four days after Obama's warning, but Representative Elijah Cummings (D-MD) sent a letter to Vice President-elect Mike Pence on November 18th asking him to address Flynn's potential conflicts of interest:
"Recent news reports have revealed that Lt. Gen. Flynn was receiving classified briefings during the presidential campaign while his consulting firm, Flynn Intel Group, Inc., was being paid to lobby the U.S. Government on behalf of a foreign government's interests," said Cummings, the ranking Democratic member of the House Committee on Oversight and Government Reform.
Cummings told Business Insider through his office at the time that he believes "the problems that have occurred with Lt. Gen. Flynn" could have been avoided had Pence heeded his warnings. Pence headed Trump's transition team, but he has insisted that he did not know about Flynn's lobbying work or conversations with the Russian ambassador prior to Flynn's ouster in February.
New Jersey Governor Chris Christie was in charge of the Trump transition team until November 11th when he lost his job and Pence took over the role.
[Christie] said recently that he believes he was fired largely because he warned transition officials against hiring Flynn as national security adviser.
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"Suffice to say, I had serious misgivings, which I think have been confirmed by the fact that he pled guilty to a felony in federal court," Christie said at a press conference last week.
Asked who he would have appointed instead of Flynn, Christie replied: "It's in about four volumes of books that were apparently thrown out the day I was terminated."
Former acting attorney general Sally Yates delivered warnings four, five, and six once Trump was in office.
Yates, the former acting attorney general, told a Senate Judiciary subcommittee earlier this year that she had had "two in-person meetings and one phone call" with the White House counsel in January to warn about Flynn's contact with Kislyak, the Russian ambassador, during the election.
President Trump fired Yates on January 30th for refusing to implement his travel ban. Flynn would not lose his job for another two weeks.
Full story: Trump officials were warned about Mike Flynn at least 6 separate times before firing him (Business Insider)