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"Insurmountable" means that something can't be overcome, as in an "insurmountable obstacle".
It is better to use "insurmountable." "Insurmountable" is the correct spelling and means something that cannot be overcome.
They are insurmountable and unsurmountable
Something which is "insurmountable" is something which can not be overcome - as in:-
Insurmountable technical problems brought an end to the project.
An obstacle may seem insurmountable.