On a Project, is it correct to have Planned Completion Date appear in two areas? I have Planned Completion Date listed in the Overview and Custom Form section?

It really depends on why you have that field there. If it was put in for a reason, you could at least rename the field to get closer to the meaning of why they are there and remove the ambiguity.

Some examples of names we've opted to use instead are "Marketing Planned Completion Date", or "Requested Completion Date" or something along those lines. We often do this because we want a record of what the requestor originally asked for, but the Planned Completion Date is ours to fill in and change as needed. i.e. "we need a historical record, so we made a second date field."

But if your custom form was built thinking to put Workfront's Planned Completion Date in a more convenient spot, then the thinking was incorrect (this is not how Workfront works), and you would be better off if you deleted the field and just coached the users on where the find the planned completion date.