Answer: Three sitting U.S. presidents, Andrew Johnson, Bill Clinton and Donald Trump have been impeached by the House of Representatives; President Trump is the only one to have been impeached twice.
Andrew Johnson was impeached for violating the Tenure of Office Act by attempting to replace Edwin Stanton, the secretary of war, while Congress was not in session and other alleged abuses of presidential power.
Bill Clinton was impeached on December 19, 1998 on grounds of perjury to a grand jury and obstruction of justice.
On 18 December 2019, Trump was impeached the first time for an abuse of power and obstruction of Congress by the House.
Trump's second impeachment: Congress adopted one article of impeachment against Trump of "incitement of insurrection", stating that he had incited the January 6 attack of the U.S. Capitol.