Delegate add or equal to?

public delegate bool InBehalfOf(string someText);

public class Test : MonoBehaviour
{
    internal static InBehalfOf inBehalfOfHandler;
    void Update()
    {
        print(inBehalfOfHandler?.Invoke("It works"));
    }
}

I have above class with a delegate declaration, somewhere earlier from different class during the runtime I call below script multiple times per second:

Test.inBehalfOfHandler = Test.DoesContainVowel;

What I noticed each time I put "equal to" it seems like it's adding methods to this delegate (I expected this line to do nothing if it's being called over again, like saying int x=5 over again will not change anything)

Test.inBehalfOfHandler += Test.DoesContainVowel;

this also behaves weird when called multiple times, what does operators = and += do for delegates?

Okay, let's say I added hundreds of the same method to a delegate then I use = operator, so it sets back my delegate to have only one method?

Test is a class, DoesContainVowel is a boolean method in that class

Test.inBehalfOfHandler is a delegate

You know it takes about 5mins to test that, and you'd get a definitive answer right?

This is what I was doing before I decided to post a question on this server. Actually @Retax you were right "perhaps there's somewhere else in your code where DoesContainWheel gets more methods somehow" my code is a mess, pressing a button switches between
Test.inBehalfOfHandler = Test.DoesContainDigit;
and
Test.inBehalfOfHandler = Test.DoesContainVowel;
but I just realized each time I press that button I'm instantiating objects of the Test class (so I keep creating new delegates for each) That was the issue, my distraction, therefore I was unable to understand why = and += behaves weird when I press the button during runtime 😄 sorry for that and thanks @Retax a lot!