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異なる is the dictionary form.
To express the idea of condition, one has to change the u into an e and add the suffix ba.
異なれば = If one differs.
How do you express the idea of a negative condition -> if one does not differ?
"if one does not 'verb'" can be expressed as : verb + ない form + ば form = verb + なければ
It is similar to the ば form conditional for い adjectives:
高い(い adj) + ば form = 高ければ
In your particular example : 異ならなければ
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