[SOLVED] I cannot find anything that is impure about my function

I cannot find anything that is impure about my function

Seriously, I'm getting desperate finding what the issue is. I can't find the answer for days!

React.StrictMode API causes our setState to be called twice, right? If it produces an error, then it means that somewhere in our setState callback is impure. So, which one is it?

setOrganization((initialValue) => {
    const newOrganization = { ...initialValue };

    const oldIssues = [...newOrganization.repository.issues.edges];
    const newIssues = [...data.data.organization.repository.issues.edges];

    newOrganization.repository.issues.edges = [...newIssues, ...oldIssues];

    return newOrganization;
});

On the first call, oldIssues is returning the expected value, for example, [{id: issue1}, {id: issue2}]. newIssues value is for example [{id: issue3}]
But on the second call, the oldIssues strangely turn into the combination of oldIssues and newIssues. (second call, oldIssues IS ALREADY [{id: issue1}, {id: issue2}, {id: issue3}]).
Making the second newOrganization.repository.issues.edges value has a doubled newIssues. [{id: issue1}, {id: issue2}, {id: issue3}, {id: issue3}]

Full script can be found here, on line 101: https://pastebin.com/ugsrBRTM

Solution

Thanks to Nick Parsons' comment above, I just know that object spreading only performs a shallow copy. All I need to do is to change the copy method to deep copy.

I found the method here: What is the most efficient way to deep clone an object in JavaScript?