My 1095-A marketplace policy number is correct- yet turbo tax says duplicate and won't accept

The software can be very picky with numbers and formats. Try deleting the number and entering it again, with dashes or without. 

If that doesn't work, you may find it useful to call in and talk to a tax support agent who can share your screen, and see exactly what the issue is. 

I had this identical issue.  The Turbo Tax Agent suggested that I somehow had two versions of Form 1095-A loaded (this is where the Marketplace Policy Number originates).  I checked under Forms and indeed this was the case.  I deleted one of the forms, and problem solved.

This worked.  thanks

Same. I had two policies in there because I had two separate plans but they can both be put on the same page. Problem solved.

I had this identical issue.  The Turbo Tax Agent suggested that I somehow had two versions of Form 1095-A loaded (this is where the Marketplace Policy Number originates).  I checked under Forms and indeed this was the case.  I deleted one of the forms, and problem solved.

Use the last 15 digits of the policy number no : or - any non-number character. And the most important thing, make sure there are no decimal places on any number in column A, B, and C, just round them up or down. I took me days to figure it out.