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:

ここで質問することではなかったらすみません。

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.
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2013-11-27T09:02:03.087
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grammar
ではなかったら and でなかったら
Textbooks vs. People.

Careful speakers would not say ではなかったら, that is for sure, but not all of us
are careful speakers and some of us will say it anyway. Textbooks need to draw
the line somewhere and I feel yours made a good decision regarding this
grammar point.

More interestingly, the sentence you gave 「ここで質問することではなかったらすみません。」 is VERY
tricky. It is actually good even with that は. It is good only because the たら-
clause is followed by すみません instead of a whole mini-sentence describing an
effect/result arising from the condition expressed in the たら-clause. For
instance, if a ほかの場所で質問します followed the たら-clause instead of a すみません, the は
would look unnecessary as far as school grammar was concerned.
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2013-11-28T00:31:52.360
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answer
There is NO such thing called "は should be omitted and should be でなかったら NOT
ではなかったら" otherwise じゃなかったら would also be ungrammatical but the fact is the
other way around. It all depends on the context and your intention to decide
whether you use は or not. Grammatically は CAN be added after で in any context
whenever you want to make it a topic except when there is already a は in the
same stucture like "でなければ" in which は already shows up in the form of ば.
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answer