Is it acceptable to duplicate the AB term in line 4 because the truth value will not change with two of that term?

That correct. One of the Boolean identities is the Idempotence Property which says that A+A=A and AA=A.

 

Another way to do this:

F = A+A'B
F = ( ( A+A'B)' )'
F = ( A'(A'B)' )'
F = ( A'(A+B') )'
F = ( A'A + A'B' )'
F = ( A'B' )'
F = A+B

 

C'+A'C can still be simplified.

Using DeMorgan's laws,
A' +C' +B'D' = (AC(B +D))'

 

Using DeMorgan's laws,
A' +C' +B'D' = (AC(B +D))'

I meant simplifying C' + A'C = C' + A'