Do not use this expression unless you are sure that both conjunctions are needed. You rarely will be.
It is planned to take the opportunity if and when it arises.
Clearly there is no justification for both words in that sentence.
Further cases will be studied if and when the material is available.
Perhaps that is one of the rare cases in which the use of the two could be justified on this ground: that if alone will not do because the writer wants to emphasise that material becoming available will be studied immediately, and when alone will not do because it is uncertain whether the material ever will be available. But it is all rather subtle.
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