Not exactly. All options are treated for de-duplication even when SHELL: is used. The effect of SHELL: is to take is argument as one option and this option will be considered for de-duplication. For example, for "SHELL:-assume realloc_lhs", CMake will consider "-assume realloc_lhs" for de-duplication.
So specify all options as part of one SHELL: will have as result to consider all your options as one option for de-duplication step.