strcat() without null character

#include <stdio.h>
#include <string.h>
int main()
{
    char str1[30] = "First";
    char str2[6] = "Second";
    strcat(str1,str2);
    printf("%s",str1);
    return 0;
}

Output is "FirstSecondFirst"
when I change str2 size 6 to 7, it functions properly.

I know that it continues to read without stopping if there is no null char, but I don’t understand why the word ‘First’ is repeated.

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