PHP - check a form for duplicate values and display a warning message

Question

I have an HTML form with a number of statements which the user can rank as 1, 2, 3 or 4. I group the statements together in clusters of 4 and the ranks should only be used once per cluster (for example 4,3,2,1 not 1,1,2,3 etc.).

I need some PHP code that will check each cluster of 4 answers and display an error message if it finds any duplicates.

This is the 1st cluster in my form (written for Contact Form 7, hence the square brackets)

1. I like pasta
[select* one-01 first_as_label class:cluster1 "--" "1" "2" "3" "4"]

2. I like potato
[select* one-02 first_as_label class:cluster1 "--" "1" "2" "3" "4"]

3. I like rice
[select* one-03 first_as_label class:cluster1 "--" "1" "2" "3" "4"]

4. I like fries
[select* one-04 first_as_label class:cluster1 "--" "1" "2" "3" "4"]

HTML:

1. I like pasta
<select name="one-01" class="cluster1" ><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option></select>

2. I like potato
<select name="one-02" class="cluster1"><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option></select>
    
3. I like rice
<select name="one-03" class="cluster1"><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option></select>

4. I like fries
<select name="one-04" class="cluster1"><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option></select>

I'm very new to PHP and I don't know the best way to treat the answers of a cluster of values as an array. (This is the best I could do. This is added to my child theme functions.php)

// Function to test for duplicate form entries
if(count(array_unique($cluster1))<count($cluster1))
{
    // Array has duplicates
    echo "You must only use each score once per cluster";
}
else
{
    // Array does not have duplicates
}

I think this php check array value for duplicate is the closest to what I'm trying to do, but I don't know how to treat all values of the same class as an array. I think there many be other errors in my implementation too.

All advice will be gratefully received by this novice!

Thanks