Conversation from the Oval Office to the FBI went, as follows?

TRUMP: "Hello, FBI? I need you to reopen the background check on Judge Kavanaugh based on the allegations of Dr Ford".

FBI: "Yes sir. Are there any people we should NOT speak with?"

TRUMP: "Yes. Judge Kavanaugh and Dr Ford".

Why does the FBI need to question them more when they already testified under oath?

Judge Kavanaugh and Dr Ford were not interviewed because they already made their statements under oath in the Senate Judiciary committee. And those statements are available to all the senators. Senators can cross check the FBI report with those statements made by Judge and Ford and come to their conclusion.

Why do Democrats want the redundant interviews to conducted?

Look, we all know you don't care one bit about seeking the truth or protecting Ford.

You just want more delays

That is because both have their statements, their testimony, under oath already. What more could they possible add to the he said she said argument? Nothing except maybe, interview my husband and my friends, they will tell you I'm not lying abut this version of my story.

First of all, Kavanaugh has no obligation to speak to the FBI.

Second, Ford already made it clear that she can't remember any significant details. Thus, there's no point in talking to her.