Google mail - Parents receiving same email more than once

[Ditto]

We use GSuite for Education. We have a group email structure where each form has a parent email group. We then have a hierarchical 'All parents' email group which contains all the parent-form email groups. A parent who has a child in two different form groups is reporting receiving 2 emails after an 'All parents' email is sent. Does anyone else hit this problem and have a solution?