Do you mean that fmincon appears to evaluate your function at the same point multiple times?
If so, how have you checked that those points are actually identical? It's possible they are very slightly different, but that the change is small enough that it doesn't change how the number is displayed in the default format if that's how you're checking. In the example below a and b are displayed the same but their stored values are not the same.
a = pi
b = a + 1e-6
b == a % false
