One of my textbooks says:
When たら used with N/なadjective+ではない, は should be omitted. So it should be でなかったら NOT ではなかったら.
This seems strange because it never explains why.
However, I do see sentences like this:
Now I am puzzled why the text makes such statements.
This textbook is widely used in my country, probably more than 100 million people are using or have used it. It should not contain so many errors.